I’m used to classifying stones by their color (i.e. purple=amethyst). How is it possible to have pink sapphires or blue topaz? Is it the internal structure of the stone?
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Usually, colors in gemstones are caused by traces of certain metal ions. However this is not always the case
Blue Sapphire are caused by small amounts of titanium and iron replacing aluminum atoms. Ruby is caused by small amounts of chromium. This is usually in the range of a few parts per million.
Chromium impurities cause the color of emeralds. The sibling of emerald, aquamarine, is the result of iron. This is perhaps not surprising since iron is one of the most common elements in the Earth’s crust.
In diamonds, small amounts of boron cause a blue color, while nitrogen causes yellow. Diamonds are composed of pure carbon, but both nitrogen and boron are adjacent to carbon on the periodic table, so they can sometimes substitute themselves into the crystal matrix of diamond.
Certain minerals are composed of colored metal ions by virtue of their composition. Malachite for example is composed of copper(+2) hydroxide-carbonate. Copper ions usually have a strong greenish or bluish color. Malachite has been used as a pigment since prehistoric times, and it has an intense color even when finely ground, something that is not true of most colored gemstones.
Vanadium minerals are often yellow or red owing to the color of the vanadate ion (VO4^3- )
Amethyst is unusual in that the color is caused by trace amounts of iron, however this normally causes a very pale greenish yellow color. The purple develops only after the quartz has been exposed to low levels of radiation for hundreds of thousands or millions of years.
You can in fact use certain citrine or smoky quartz varieties and expose them to a source of intense radiation for several hours, and the amethyst color will develop. This also happens with some kinds of glass. (Quartz sand is the main ingredient in most types of glass.)
This irradiating process can also be used to produce semi-artificial colors in diamonds, ranging from pink to light green (depending on the diamonds’ composition.) Naturally occurring pink diamonds are rare, but probably likewise a result of exposure to radioactive minerals.
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