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Orpiment and realgar are ores of arsenic. The main deposits for these two minerals in the US are in Mercer, Utah, and Manhattan, Nevada. They are no longer mined due to the fact that arsenic is rarely used in modern times. It was originally used in a great variety of items, including paint pigments for the colours green and red, as well as in fireworks and poison. It was also used by society ladies in the beginning of the 20th Century because a little arsenic would pale the skin and make the lips rosy red - the fashion of the time1. Recently, arsenic has been considered 'a pollutant' and 'the next asbestos'2.
The only major modern uses for arsenic are poison, which by now is mostly accidental, and for collectors' samples. Larger samples of both orpiment and realgar can be quite valuable, especially if they are crystalline. Orpiment is almost always amorphous (without crystal form) and yellow, and realgar occasionally forms tabular red crystals. It's actually very pretty if you have a good sample, and the Chinese used to carve it. However, very few of those carvings have survived because when exposed to light, orpiment and realgar decompose.
The two minerals are found in veins of gold, silver, and other precious metals. When these metals are mined, the arsenic ores that are discarded get into the ground water and cause all kinds of horrible illnesses, including cancer. Many companies that mine these metals, especially in the western US, are very concerned about the issue.
Recently, a device was invented that uses the sun's UV rays to decompose arsenic into its less toxic forms, which could potentially save companies millions of dollars. It uses only water and natural sunlight, which are much cheaper than other methods of removing toxins.