You may be thinking of gemstones and industrial diamonds. Gemstones represent about 20% of all diamonds mined, and the remainder is industrial diamonds.
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You are probably referencing industrial diamonds, of which are are many, many different types, sizes and descriptions. You can read about the basic categories, below.
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About 20% of all diamonds mined are gem-quality diamonds.
Yes, diamonds can cut glass, because diamonds are harder than glass.
The plural possessive is diamonds'.