Diamonds are almost impossible to detect directly because they are so rare: very rich kimberlite pipes, the routes through which...

GMAT Reading Comprehension : (RC) Questions

Source: Official Guide

Reading Comprehension

Physical Sciences

HARD

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Notes

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Diamonds are almost impossible to detect directly because they are so rare: very rich kimberlite pipes, the routes through which diamonds rise, may contain only three carats of diamonds per ton of kimberlite. Kimberlite begins as magma in Earth's mantle (the layer between the crust and the core). As the magma smashes through layers of rock, it rips out debris, creating a mix of liquid and solid material. Some of the solid material it brings up may come from a so-called diamond-stability field, where conditions of pressure and temperature are conducive to the formation of diamonds. If diamonds are to survive, though, they must shoot toward Earth's surface quickly. Otherwise, they revert to graphite or burn. Explorers seeking diamonds look for specks of "indicator minerals" peculiar to the mantle but carried up in greater quantities than diamonds and eroded out of kimberlite pipes into the surrounding land. The standard ones are garnets, chromites, and ilmenites. One can spend years searching for indicators and tracing them back to the pipes that are their source; however, 90 percent of kimberlite pipes found this way are barren of diamonds, and the rest are usually too sparse to mine.

In the 1970's the process of locating profitable pipes was refined by focusing on the subtle differences between the chemical signatures of indicator minerals found in diamond-rich pipes as opposed to those found in barren pipes. For example, G10 garnets, a type of garnet typically found in diamond-rich pipes, are lower in calcium and higher in chrome than garnets from barren pipes. Geochemists John Gurney showed that garnets with this composition were formed only in the diamond-stability field; more commonly found versions came from elsewhere in the mantle. Gurney also found that though ilmenites did not form in the diamond-stability field, there was a link useful for prospectors: when the iron in ilmenite was highly oxidized, its source pipe rarely contained any diamonds. He reasoned that iron took on more or less oxygen in response to conditions in the kimberlitic magma itself—mainly in response to heat and the available oxygen. When iron became highly oxidized, so did diamonds; that is, they vaporized into carbon dioxide.

Ques. 1/5

The primary purpose of the passage is to

discuss an objection to Gurney's theories about the uses of indicator minerals

explore the formation of diamonds and the reasons for their scarcity

analyze the importance of kimberlite pipes in the formation of diamonds

define the characteristics of indicator minerals under differing conditions

explain a method of determining whether kimberlite pipes are likely to contain diamonds

Solution

1. Passage Analysis:

Progressive Passage Analysis

Text from Passage	Analysis
Diamonds are almost impossible to detect directly because they are so rare: very rich kimberlite pipes, the routes through which diamonds rise, may contain only three carats of diamonds per ton of kimberlite.	What it says: Finding diamonds is extremely hard because there are so few of them. Even in the best locations (kimberlite pipes), there are only tiny amounts. What it does: Introduces the central problem - diamond detection difficulty Source/Type: Factual statement with concrete example Connection to Previous Sentences: This is the opening - establishes the foundation of our analysis Visualization: Imagine a massive dump truck carrying 2,000 pounds of rock material, but hidden somewhere in that enormous pile are only 3 tiny diamonds the size of peas Reading Strategy Insight: This sentence immediately tells us what the entire passage will be about - the challenge of finding diamonds. Start with this clear framework in mind. What We Know So Far: Diamonds are rare and hard to find; they come through kimberlite pipes What We Don't Know Yet: How diamonds form, why they're in kimberlite pipes, how people actually search for them
Kimberlite begins as magma in Earth's mantle (the layer between the crust and the core).	What it says: Kimberlite starts as melted rock deep inside the Earth What it does: Begins explaining the geological process behind diamond formation Source/Type: Factual/geological explanation Connection to Previous Sentences: This builds on Sentence 1 by explaining what kimberlite actually is. Sentence 1 mentioned "kimberlite pipes" but didn't explain what kimberlite was - now we're getting that background. Visualization: Picture the Earth like a three-layer cake: the thin outer crust we live on, the thick middle layer (mantle) where it's so hot that rock melts into liquid magma, and the core at the center Reading Strategy Insight: The passage is being helpful here - it's defining the technical term "kimberlite" and even clarifying what "mantle" means in parentheses
As the magma smashes through layers of rock, it rips out debris, creating a mix of liquid and solid material.	What it says: The liquid rock shoots upward violently, tearing apart rock layers and collecting pieces along the way What it does: Continues the geological explanation of how kimberlite moves and changes Source/Type: Factual description of geological process Connection to Previous Sentences: This directly continues from Sentence 2. We learned kimberlite starts as magma in the mantle - NOW we're learning what happens next: it moves upward destructively. Visualization: Imagine a powerful underground geyser of liquid rock shooting upward through layers of solid rock like a drill, but instead of making a clean hole, it's smashing and collecting rock fragments, creating a violent mixture of liquid and solid chunks Reading Strategy Insight: Notice the logical sequence: Step 1 (starts as magma) → Step 2 (moves upward violently). The passage is building a clear timeline.
Some of the solid material it brings up may come from a so-called diamond-stability field, where conditions of pressure and temperature are conducive to the formation of diamonds.	What it says: Sometimes the rock pieces that get picked up come from a special zone where diamonds can actually form What it does: Introduces the key concept of where diamonds actually originate Source/Type: Factual explanation with technical term introduction Connection to Previous Sentences: This builds on Sentence 3's mention of "solid material" being ripped out. Now we learn that SOME of this material might be special - it might include diamonds from the diamond-formation zone. Visualization: Picture the upward-moving magma passing through different neighborhoods underground. Most neighborhoods are regular rock, but occasionally it passes through a special "diamond district" where the pressure and heat are just right for diamond formation Reading Strategy Insight: Key insight: This is where diamonds come from! They're not created by the kimberlite process - they already exist in special zones and get picked up by the kimberlite. What We Know So Far: Diamonds are rare; kimberlite starts as magma, moves upward violently, and sometimes picks up diamonds from special pressure/temperature zones What We Don't Know Yet: What happens to diamonds after they're picked up, how people search for them
If diamonds are to survive, though, they must shoot toward Earth's surface quickly. Otherwise, they revert to graphite or burn.	What it says: Diamonds are fragile during this upward journey - they need to reach the surface fast or they'll be destroyed What it does: Introduces a crucial constraint/vulnerability in the diamond formation process Source/Type: Factual statement about diamond stability Connection to Previous Sentences: This adds a critical "but" to our story. Sentences 2-4 explained how diamonds get picked up from the diamond-stability field, but NOW we learn there's a catch - they might not survive the journey. Visualization: Think of diamonds as ice cubes that the kimberlite magma has picked up. If the journey to the surface takes too long, the "ice cubes" melt (revert to graphite) or completely vaporize (burn) Reading Strategy Insight: This explains even more why diamonds are so rare from Sentence 1! Not only are the diamond-zones rare, but even when diamonds get picked up, many don't survive the trip.
Explorers seeking diamonds look for specks of "indicator minerals" peculiar to the mantle but carried up in greater quantities than diamonds and eroded out of kimberlite pipes into the surrounding land.	What it says: Since diamonds are so rare and hard to find directly, diamond hunters look for other minerals that come from the same deep places but are much more common What it does: Introduces the practical solution to the detection problem established in Sentence 1 Source/Type: Description of industry practice/methodology Connection to Previous Sentences: This directly addresses the opening problem! Sentence 1 said diamonds are "almost impossible to detect directly" - THIS sentence explains the workaround solution that explorers actually use. Visualization: Imagine you're looking for a friend's house in a neighborhood, but the house numbers are too small to see. Instead, you look for the distinctive red mailbox you know is nearby - it's much easier to spot and will lead you to the right area. Indicator minerals are like that red mailbox. Reading Strategy Insight: Feel relieved here - this is the practical answer to our opening challenge! The passage is moving from "here's the problem" to "here's how people actually solve it."
The standard ones are garnets, chromites, and ilmenites.	What it says: The main types of indicator minerals are garnets, chromites, and ilmenites What it does: Provides specific examples of the indicator minerals mentioned in the previous sentence Source/Type: Factual list/categorization Connection to Previous Sentences: This simply gives us the specific names for the "indicator minerals" mentioned in Sentence 6. This is pure clarification - no new concepts, just filling in details. Visualization: Think of these as three different types of "red mailboxes" - garnets, chromites, and ilmenites are the three main types of easy-to-find minerals that signal "diamonds might be nearby" Reading Strategy Insight: This is a simple supporting detail. Don't overthink it - just note that there are three main types and move on. The specific names matter less than understanding they're indicators.
One can spend years searching for indicators and tracing them back to the pipes that are their source; however, 90 percent of kimberlite pipes found this way are barren of diamonds, and the rest are usually too sparse to mine.	What it says: Even the indicator method is difficult and usually disappointing - it takes years of work, and 90% of the time you find nothing valuable What it does: Shows the limitations of the solution introduced in Sentences 6-7 Source/Type: Factual statement about success rates in diamond exploration Connection to Previous Sentences: This adds a sobering reality check to the "solution" presented in Sentences 6-7. The passage is being honest: even the best available method usually fails. Visualization: Imagine spending years following those "red mailboxes" back to houses, only to discover that 9 out of 10 houses you find are completely empty, and the 10th house has so few valuable items it's not worth the effort Reading Strategy Insight: This reinforces the rarity theme from Sentence 1 but with specific numbers. The passage is building a consistent picture: diamond finding is extremely challenging at every level. What We Know So Far: Diamonds are rare, form deep underground, often don't survive the trip up, and even the indirect detection method (indicator minerals) fails 90% of the time What We Don't Know Yet: Whether there are better methods or improvements to this process
In the 1970's the process of locating profitable pipes was refined by focusing on the subtle differences between the chemical signatures of indicator minerals found in diamond-rich pipes as opposed to those found in barren pipes.	What it says: In the 1970s, scientists improved the success rate by studying the chemical differences between indicator minerals that come from diamond-rich areas versus those from diamond-poor areas What it does: Introduces an improvement/refinement to the previously described method Source/Type: Historical account of methodological advancement Connection to Previous Sentences: This directly addresses the poor success rate mentioned in Sentence 8! Instead of just finding any indicator minerals, scientists learned to distinguish between "good" and "bad" indicators. Visualization: Going back to our mailbox analogy - scientists learned to tell the difference between red mailboxes in wealthy neighborhoods (indicating valuable houses) versus identical red mailboxes in poor neighborhoods (indicating empty houses) Reading Strategy Insight: This is great news after the discouraging Sentence 8! The passage is showing us progress and improvement, not just more problems.
For example, G10 garnets, a type of garnet typically found in diamond-rich pipes, are lower in calcium and higher in chrome than garnets from barren pipes.	What it says: Here's a specific example: G10 garnets (found near diamonds) have different chemical composition - less calcium, more chrome - than regular garnets What it does: Provides a concrete example of the "chemical differences" mentioned in the previous sentence Source/Type: Specific factual example of chemical analysis Connection to Previous Sentences: This is a direct example illustrating Sentence 9's "subtle differences between chemical signatures." The passage is being helpful by giving us a concrete case instead of leaving the concept abstract. Visualization: Think of two identical-looking red mailboxes, but when you analyze the paint, one has a different chemical formula - that slight difference tells you which neighborhood is more likely to have valuable houses Reading Strategy Insight: Don't get lost in the chemical details (calcium/chrome). The key point is that scientists can now distinguish between "good" and "bad" indicator minerals by their chemical makeup.
Geochemists John Gurney showed that garnets with this composition were formed only in the diamond-stability field; more commonly found versions came from elsewhere in the mantle.	What it says: Researcher John Gurney proved that these special garnets (G10s) only form in the same deep zones where diamonds form, while regular garnets come from different areas What it does: Provides the scientific explanation for why the chemical difference matters Source/Type: Research finding/scientific evidence Connection to Previous Sentences: This explains WHY the G10 garnets from Sentence 10 are better indicators. It connects back to Sentence 4's "diamond-stability field" - the G10s come from that same special zone! Visualization: The special red mailboxes (G10 garnets) only exist in the wealthy neighborhood (diamond-stability field), while regular red mailboxes are scattered throughout all neighborhoods (elsewhere in the mantle) Reading Strategy Insight: This is a satisfying "aha" moment - the passage is connecting earlier concepts (diamond-stability field) with current information (G10 garnets). Feel confident that the pieces are fitting together!
Gurney also found that though ilmenites did not form in the diamond-stability field, there was a link useful for prospectors: when the iron in ilmenite was highly oxidized, its source pipe rarely contained any diamonds.	What it says: Gurney discovered that ilmenites (another indicator mineral) don't come from diamond zones, but they still help: when iron in ilmenites is heavily oxidized, that usually means no diamonds nearby What it does: Introduces a different type of indicator - one that tells you where NOT to look Source/Type: Additional research finding from the same scientist Connection to Previous Sentences: This expands Gurney's research beyond garnets to ilmenites (mentioned back in Sentence 7 as one of the three main indicator types). Unlike garnets, this is a negative indicator - it tells you where diamonds WON'T be. Visualization: Some mailboxes have rust stains (highly oxidized iron) - when you see those rusty mailboxes, you know you're definitely in the wrong neighborhood for what you're looking for Reading Strategy Insight: The passage is showing us that indicator minerals work in two ways: some point toward diamonds (G10 garnets) and some point away from diamonds (oxidized ilmenites). Both are useful for prospectors.
He reasoned that iron took on more or less oxygen in response to conditions in the kimberlitic magma itself—mainly in response to heat and the available oxygen.	What it says: Gurney figured out that iron oxidation levels depend on the conditions in the kimberlite magma - especially temperature and oxygen availability What it does: Explains the mechanism behind the oxidation observation from the previous sentence Source/Type: Scientific reasoning/explanation of cause Connection to Previous Sentences: This explains WHY iron oxidation matters (from Sentence 12). Gurney wasn't just observing a pattern - he understood the underlying chemical process. Visualization: Think of iron as a sponge that absorbs more or less oxygen depending on the environment it's in - hotter conditions with more available oxygen create rustier (more oxidized) iron Reading Strategy Insight: This is background explanation - useful to understand but not a new main concept. The key point remains: oxidized iron indicates no diamonds.
When iron became highly oxidized, so did diamonds; that is, they vaporized into carbon dioxide.	What it says: In conditions that create highly oxidized iron, diamonds also get oxidized, which means they completely disappear by turning into gas What it does: Completes the scientific explanation by showing what happens to diamonds in high-oxidation conditions Source/Type: Scientific explanation of diamond destruction process Connection to Previous Sentences: This beautifully connects multiple earlier concepts! It relates to Sentence 5 (diamonds can "burn"), Sentence 12 (oxidized iron means no diamonds), and Sentence 13 (conditions in magma affect oxidation). Visualization: In the rusty mailbox neighborhoods (high oxidation), any diamonds that might have existed literally evaporated into invisible gas before reaching the surface Reading Strategy Insight: This is the satisfying conclusion that ties together the scientific reasoning. The passage has moved from observation (oxidized iron = no diamonds) to complete explanation (why that relationship exists). Final Summary - What We Know: Diamonds are rare, hard to find directly, come from deep stability fields, often don't survive the journey up, can be located indirectly through indicator minerals, and the chemical signatures of these indicators (especially oxidation levels) can predict success or failure in diamond prospecting.

# 2. Passage Summary:

Author's Purpose:

To explain why diamonds are so hard to find and how scientists have improved methods for locating them by studying the chemical clues left behind by other minerals.

Summary of Passage Structure:

In this passage, the author walks us through both the natural process that makes diamonds rare and the human solutions developed to find them:

First, the author establishes the core problem - diamonds are extremely rare and almost impossible to detect directly, even in the best locations.
Next, the author explains the geological process behind this rarity, showing how diamonds form deep underground, get picked up by rising magma, and often don't survive the violent journey to the surface.
Then, the author describes how diamond hunters work around this problem by looking for indicator minerals that are more common but come from the same deep sources, though this method still fails 90% of the time.
Finally, the author shows how scientists in the 1970s improved success rates by learning to read the chemical signatures of these indicator minerals, using researchers like John Gurney's discoveries to distinguish between promising and worthless locations.

Main Point:

While diamonds remain incredibly difficult to find due to their rarity and the harsh conditions they face during formation, scientists have significantly improved diamond prospecting by learning to interpret the chemical clues in indicator minerals that reveal whether a location is likely to contain diamonds or not.

3. Question Analysis:
This question asks us to identify the primary purpose of the passage – essentially, what is the author's main goal in writing this piece? We need to look at the overall structure and flow of ideas rather than getting caught up in specific details.

Connecting to Our Passage Analysis:
Our passage analysis reveals a clear progression:

The author establishes that diamonds are extremely rare and hard to detect directly.
The author explains the geological process of diamond formation and transportation through kimberlite pipes.
The author describes how explorers work around detection problems by using indicator minerals.
The author shows how scientists improved success rates by studying chemical signatures of these indicators.

The passage consistently returns to the theme of kimberlite pipes throughout – they are introduced as “routes through which diamonds rise” in sentence 1, explained as originating from mantle magma in sentences 2–4, described as the source that indicator minerals get “eroded out of” in sentence 6, and repeatedly referenced as either “diamond-rich pipes” or “barren pipes” in the methodology discussion.

Prethinking:
The passage seems primarily focused on explaining how scientists determine whether kimberlite pipes contain diamonds. While it touches on diamond formation, scarcity, and indicator mineral characteristics, these topics all serve the larger purpose of explaining the methodology for evaluating pipe potential. The 1970s refinements by Gurney and others represent the culmination of this methodological explanation.

Answer Choices Explained

discuss an objection to Gurney's theories about the uses of indicator minerals

Why It's Wrong:

The passage presents Gurney's theories as accepted scientific findings, not controversial ideas facing objections
No alternative viewpoints or criticisms of Gurney's work are mentioned
The tone is explanatory and supportive of the scientific advances described

explore the formation of diamonds and the reasons for their scarcity

Why It's Wrong:

analyze the importance of kimberlite pipes in the formation of diamonds

Why It's Wrong:

Kimberlite pipes are described as routes through which diamonds rise, but their importance in diamond formation is not analyzed in depth.
The passage focuses on evaluating whether pipes contain diamonds, not on their geological role in forming diamonds.
The detailed discussion is about indicator minerals and chemical signatures, not about how kimberlite pipes create diamonds.

define the characteristics of indicator minerals under differing conditions

Why It's Wrong:

Defining characteristics is a supporting detail, not the main purpose
The passage mentions indicator mineral characteristics to support the larger point about pipe evaluation
The focus is on how these characteristics help distinguish promising from unpromising locations

explain a method of determining whether kimberlite pipes are likely to contain diamonds

Why It's Right:

The passage describes how explorers use indicator minerals and later their chemical signatures to evaluate pipes.
The second paragraph, especially, explains the refined 1970s method: using G10 garnets and ilmenite oxidation to judge whether a pipe is diamond-rich or barren.
From start to finish, the passage is about how to tell which kimberlite pipes are likely to contain diamonds, i.e., the method of determination—exactly what this choice says.

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