Chrysoprase

Chrysoprase forms at shallow depths where nickel-bearing ultramafic rocks such as peridotite, dunite and serpentinite are deeply weathered, filling cavities, veins and fractures that are usually only a few centimeters wide, so it is mined by the kilogram. The Szklary deposit in Lower Silesia, Poland, was worked intermittently from the Middle Ages until 1980. Australia is now the leading producer, notably Marlborough in Queensland and the Eastern Goldfields of Western Australia (for example Yerilla and Marshall Pool). Near Haneti in central Tanzania, the Iyobo Mountain mine typically yields 7–10 tonnes of mine-run material a year, of which only 3–5% is high quality. Other sources include Kazakhstan, Brazil, Russia and California.

Much ordinary chalcedony and agate is dyed green to imitate chrysoprase and is often sold incorrectly as "chrysoprase" or "chrysoprase chalcedony"; the correct name is dyed green chalcedony. Dyed material tends to have perfectly even color and clarity and often shows agate banding, which chrysoprase rarely displays; the Chelsea filter (dyed green chalcedony appears pink) and the spectroscope help separate them. Green prase opal looks similar but has a lower refractive index (about 1.45 vs 1.55) and specific gravity (about 2.11 vs 2.60). Green glass and chromium-colored chalcedony (mtorolite) are other look-alikes. "Lemon chrysoprase" is a magnesite rock that is softer (about 4) and heavier (about 3.0) and effervesces in warm acid.

The color of some chrysoprase is unstable: prolonged sunlight or heat can make it fade, so avoid heating the stone during jewelry repair and store it away from direct sunlight and heat sources. Faded chrysoprase has sometimes been recorded to regain its color when stored in very humid conditions.

Chrysoprase was known to the ancient Greeks and Romans, though few pieces have been found among their artifacts, and it has been carved into cameos and intaglios since at least the Roman Empire. It first became a popular gem in Europe after deposits in Poland (then German Silesia) were discovered in the 1700s.

Chrysoprase was traditionally associated with curing disorders now linked to uric acid, such as gout and kidney stones. It is also considered one of the stones in the foundation of the Heavenly City described in the Bible, and in some modern birthstone schemes it is regarded as a birthstone for May.