Gems

Gems (or Gemstones) were minerals especially beautiful and desired by peoples of Arda. Many were crystalline and transparent, though some varieties were opaque. Most could be worked to accent their beauty.
== Gemstones referenced in Tolkien's Mythology ==
* Crystals - Probably a kind of transparent quartz. The Mithril mail Thorin gave to Bilbo came with a belt studded with crystal.
* Diamonds - Highly compressed crystalline carbon. Nenya was set with "adamant," an old word meaning diamond.
* Emeralds - A crystalline green gem, also known as beryl. A favorite amongst Elves in Middle-earth.
* Pearls - Organically created mineral from shellfish. Nimphelos was given by Thingol to the Dwarves of Belegost for building Menegroth.
* Rubies - A red variant of the same mineral (corundum) as sapphire. Narya the Elf-ring of Fire, had a Ruby.
* Sapphires - A blue crystalline gem mounted on the Elf-Ring Vilya.
* Silmarils - artificial gemstones, perhaps, filled magically with the light of the Two Trees of Valinor.
* The Ñoldor being masters of crafts and metallurgy had and a great horde of Gemstones many of which were stolen by Melkor after he left Aman.
Each of the Rings of Power was set with an identifying gemstone, but these are not specified. There are several alternate colors of sapphire, colored diamonds and other shades of beryl as well. Some opaque stones, such as jade and turquoise can be used as gemstones in jewelry.
=== Of unknown type ===
* Arkenstone - Prized jewel of the Dwarves of Erebor
* Elfstone - Green gem carried by Aragorn II Elessar
* Star of Elendil - Worn by Elendil and his heirs