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Colorless diamonds can command high prices as gemstones. A type of

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jabhatta2

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27 Dec 2022, 21:08

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Hi avigutman - can we say there are 4 types of diamonds ?

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(a) naturally colorless

(b) colored diamonds that are treatable and worthwhile to be treat-able

(c) colored diamonds that are treatable but not worthwhile to be treat-able

(d) colored diamonds that are NOT Treat-able.

(d) is inferable probably because of the sentence ". A type of less valuable diamonds can be treated to remove all color"

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Would you agree with this is the ratio split ?

lets say that a total of 100 diamonds are extracted yearly

(b) + (c) sum upto 2

The other 98 is spread between (a) and (d) ?

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avigutman

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28 Dec 2022, 07:15

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jabhatta2

Hi avigutman - can we say there are 4 types of diamonds ?

Quote:

Yes. The key is the 'that'-modifier of this sentence, jabhatta2:

Quote:

only 2 percent of diamonds mined are of the colored type that can be successfully treated

The implication is that 98% of diamonds mined are either colorless or they're colored but can't be successfully treated.
This GMAT problem plays on the fact that we have no information about the ratio of colorless diamonds mined to colored-but-can't-be-successfully-treated diamonds mined.

jabhatta2

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28 Dec 2022, 07:31

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avigutman

jabhatta2

Hi avigutman - can we say there are 4 types of diamonds ?

Quote:

Yes. The key is the 'that'-modifier of this sentence, jabhatta2:

Quote:

only 2 percent of diamonds mined are of the colored type that can be successfully treated

Hi avigutman - do you think one needs to go down to such 'granular' level (specifically - 4 types of diamonds) when doing such a CR problem ?

it took me 10 mins to infer that the argument is referring to 4 types of diamonds (a/b/c and d)

once i understood there were 4 types of diamonds, then i could see that we didn't know the ratio between (a) vs (d) as part of the 98 specifically.

Were you able to go into such granularity in 2 mins OR do you solve this CR questions using perhaps more "High level MECE groups"

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28 Dec 2022, 07:54

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jabhatta2

jabhatta2 I took a very long pause after the 'that'-modifier. In that pause, I wondered about the implication for the 98%, and realized that it contains both colorless and untreatable diamonds. Then I wondered about their ratio, because the former are WAY more valuable than the latter - so if I'm the mining company I'm hoping for a high ratio - but I suspect it's probably quite low, unfortunately. After all that wondering, by the time I got to the answer choices, (B) looked really good.

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Premise: only 2 percent of diamonds mined are of the colored type that can be successfully treated, and many of those are of insufficient quality to make the treatment worthwhile

Conclusion: the vast majority of colorless diamonds sold by jewelers are naturally colorless.

The argument makes the assumption that there are negligible amount of fake colorless diamonds (compared to overall). The vast majority of colorless diamonds sold by jewelers are naturally colorless. We can weaken the argument by dismiss this assumption: what if the amount of fakes, albeit small %, are still a significant amount compared to overall. Let’s say 1000 diamonds mined, <20 can be successfully treated, let’s say 5 ended up being successful. What if there are only 3 naturally colorless diamonds. Then the fake would still be the majority.

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The Argument
Premise 1: Colorless diamonds command high prices as gemstones.
Premise 2: Some less-valuable diamonds (of a colored type) can be treated to remove all color, and only sophisticated tests can tell them apart from naturally colorless ones.
Premise 3: However, only 2 percent of diamonds mined are of the colored type that can be successfully treated—and many of those are of insufficient quality to make treatment worthwhile.
Conclusion: Therefore, the vast majority of colorless diamonds sold by jewelers are naturally colorless.
What the Argument Assumes
The argument concludes that most colorless diamonds sold are naturally colorless because the supply of diamonds that can be treated is very small (only 2% of mined diamonds, with many being unsuitable for treatment). To make this leap, the argument assumes that the small percentage of treatable diamonds (even if treated) contributes negligibly to the supply of colorless, gemstone-quality diamonds.

Identifying the Flaw
The flaw in the reasoning is that the argument fails to consider how the rarity of naturally colorless, gemstone-quality diamonds compares with the rarity of those diamonds that can be successfully treated. In other words, even if only 2 percent of all diamonds are treatable, the argument does not provide any information about the relative frequency or availability of naturally colorless, high-quality diamonds versus those produced by treatment.

Without combining the information about the rarity of treatable colored diamonds with information about the rarity (or abundance) of naturally colorless, gemstone-quality diamonds, one cannot validly conclude that most colorless diamonds sold are natural rather than treated.

Matching with the Answer Choices
(A) Discusses comparisons of price for different uses, which is not the point of the argument.
(C) Concerns the possibility of using colored diamonds as gemstones without treatment. This is not at issue in the argument.
(D) Talks about the method for making colorless diamonds, but the argument isn’t about method exclusivity.
(E) Mentions customer difficulty in distinguishing diamonds, but that fact is already stated and is not central to the flaw.
(B) states: "information about the rarity of treated diamonds is not combined with information about the rarity of naturally colorless, gemstone diamonds." This is exactly the oversight in the argument. The argument uses the 2% figure for treatable diamonds without comparing it to the actual frequency of naturally colorless, gemstone-quality diamonds that are sold.

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31 Aug 2025, 06:48

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How can we assume that rarity is associated with number and this is giving a number ratio?

KarishmaB

Colourless diamonds are expensive.
Some coloured diamonds can be made colourless (and presumably sold at high price)
Only 2% of diamonds mined (including coloured and colourless) are treatable and even fewer (say just 1% of all diamonds mined) are good enough to make it worthwhile.

Conclusion: Vast majority of colourless diamonds sold by jewellers are naturally colourless.

There is a problem here. When we talk about the numbers 2% and 1%, we are talking about the set of all diamonds mined. In our conclusion, we are talking about the set of diamonds that are sold by the jewellers. Without knowing how big the set of diamonds sold by jewellers is, we cannot make the comparison.

Say total diamonds mined = 1000
Out of these, say 100 are naturally colourless.
Another 10 (1%) are coloured but profitably treatable.
So the jewellers could be selling 110 diamonds out of which the vast majority (100) could be naturally colourless.

Say total diamonds mined = 1000
Out of these, say 5 are naturally colourless.
Another 10 (1%) are coloured but profitably treatable.
So the jewellers could be selling 15 diamonds out of which the vast majority (10) are coloured but treated.

Hence, whether our conclusion works or not, depends on what % of all diamonds mined are naturally colourless.

(A) comparisons between the price diamonds command as gemstones and their value for other uses are omitted
Other uses are irrelevant.

(B) information about the rarity of treated diamonds is not combined with information about the rarity of naturally colorless, gemstone diamonds
Correct. As discussed above, we need info on rarity of naturally colourless diamonds to reach our conclusion.

(C) the possibility that colored diamonds might be used as gemstones, even without having been treated, is ignored
We are talking about colourless diamonds as gemstones. Irrelevant to our conclusion.

(D) the currently available method for making colorless diamonds from colored ones is treated as though it were the only possible method for doing so
We are talking about the current scenario only and the currently available method only.

(E) the difficulty that a customer of a jeweler would have in distinguishing a naturally colorless diamond from treated one is not taken into account
The argument says that only sophisticated tests can differentiate. This doesn't have anything to do with our conclusion.

Answer (B)

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Hi Karishma,

The argument says 2% of diamonds mined are of the colored type that can be successfully treated. However, as per your analysis, 2% of diamonds mined (including coloured and colourless) are treatable which I can’t seem to agree with. Could you pls clarify

KarishmaB