I think it might have something to with the crystalline structure, but you can have metal crystals and those aren’t clear.
In: Physics
Mineral/rock/metal are scientific terms while gemstone is more of a cultural term. A gemstone is a rock or mineral that people use in jewelry. Most people like having clear and shiny gemstones, but there are some opaque gemstones like pearl, turquoise, and lapis lazuli.
There are some clear minerals that you rarely see as gemstones. Calcite is perhaps the most common example. If I had to guess, the biggest problem with calcite is that it has a low hardness value, which means that it is really easy to scratch. (For example, I’m pretty sure that the metal from the ring could scratch a calcite stone.) Gemstones like diamonds, rubies, emeralds, topaz, quartz, etc., all have really high hardness, which makes them resistant to scratching. There are probably other reasons too.
The color of a stone and whether light passes through it is related to the chemical structure of the crystal. Impurities in the crystal can actually add color to an otherwise clear stone. ~~For example rubies, sapphires, and emeralds are all the same type of mineral (corundum) with different types of impurities. It is possible for a corundum stone to have too many impurities and be too opaque for use as a gemstone.~~ Impurities are also why there are different colors of diamonds, which are normally clear.
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