Gemstones are described by specific gravity, hardness, and color. These are all features of minerals.
Specific Gravity
Specific gravity is a measure of how much heavier a mineral is than an equal amount of water. Tourmaline has a specific gravity of 3.03 to 3.25. This means that it is a bit more than three times as heavy as water.
Hardness
Hardness is the resistance of a mineral to scratching. Geologists use a measurement called Mohs’ scale to describe the hardness of a mineral.
You can see that topaz has a hardness of 8 on the scale. This means that a topaz is harder than anything except a sapphire or a diamond
Color
How would you describe turquoise? You’d probably say it’s blue-green. Gemstones are described according to their color because it’s a useful characteristic. Diamonds are clear, but can be tinted yellow, pink, or even black from the presence of other minerals.
Rare and highly valuable diamonds in colors of blue, yellow, red and pink occur only in one in every 100,000 diamonds
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