Arne Adolfsen has posted on Facebook about a forthcoming book on emeralds, which looks gorgeous. An illustration:


A story here on the book, Emerald: A Glittering Visual History Of Emeralds, which is scheduled to be published on February 16th.

Emeralds are one of the four classic major gemstones, the other three being diamonds, sapphires, and rubies.

Diamond is a form of carbon.

Emerald is a gemstone, and a variety of the mineral beryl (Be3Al2(SiO3)6) colored green by trace amounts of chromium and sometimes vanadium. (Wikipedia link)

Other varieties of beryl:

Aquamarine (from Latin: aqua marina, “water of the sea”) is a blue or turquoise variety of beryl.

… Clear yellow beryl, such as that occurring in Brazil, is sometimes called aquamarine chrysolite.The deep blue version of aquamarine is called maxixe. (Wikipedia link)

Then rubies and sapphires:

A ruby is a pink to blood-red colored gemstone, a variety of the mineral corundum (aluminium oxide). The red color is caused mainly by the presence of the element chromium. Its name comes from ruber, Latin for red. Other varieties of gem-quality corundum are called sapphires. (Wikipedia link)

Much nomenclatural complexity here. Red corundrum crystals are rubies, but other colors (blue, notably, but there are others) are sapphires.

Then there are semi-precious stones and other crystals, notably quartz:

Silicon dioxide, also known as silica (from the Latin silex), is a chemical compound that is a dioxide of silicon with the chemical formula SiO2. It has been known since ancient times. Silica is most commonly found in nature as quartz, as well as in various living organisms, (Wikipedia link)

And that brings us to non-crystalline silica:

Opal is a hydrated amorphous form of silica; its water content may range from 3% to 21% by weight, but is usually between 6% to 10%. Because of its amorphous character it is classed as a mineraloid, unlike the other crystalline forms of silica which are classed as minerals. (Wikipedia link)

Note the mineral vs. mineraloid distinction — not one made in ordinary language.

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