Nephrite

Nephrite
Nephrite from Jordanów Śląski (Poland)
General
Category	Mineral
Formula (repeating unit)	Ca2(Mg,Fe)5Si8O22(OH)2
Crystal system	monoclinic
Identification
Color	Translucent to opaque and often mottled. Light to dark green, yellow to brown, white, gray, black.
Crystal habit	massive
Fracture	splintery to granular
Mohs scale hardness	6 – 6.5
Luster	dull
Specific gravity	2.95 (+.15, -.05)
Polish luster	vitreous to greasy
Optical properties	Double refractive with anomalous aggregate reaction
Refractive index	1.606 – 1.632 (+.009, -.006)
Birefringence	usually not detectable
Pleochroism	none
Ultraviolet fluorescence	inert
Absorption spectra	Vague line may be present at 500 nm, but rarely any lines. Rarely, in stones of exceptional gem quality, vague lines in the red part of the spectrum may be seen.

Nephrite is a variety of the calcium and magnesium and iron-rich amphibole minerals tremolite or actinolite (aggregates of which also make up one form of asbestos). The chemical formula for nephrite is Ca₂(Mg, Fe)₅Si₈O₂₂(OH)₂. It is one of two different mineral species called jade. The other mineral species known as jade is jadeite, which is a variety of pyroxene. While nephrite jade possess mainly grays and greens (and occasionally yellows, browns or whites), jadeite jade, which is rarer, can also contain blacks, reds, pinks and violets. Nephrite jade is an ornamental stone, used in carvings, beads, or cabochon cut gemstones.

Nephrite can be found in a translucent white to very light yellow form which is known in China as mutton fat jade, in an opaque white to very light brown or gray which is known as chicken bone jade, as well as in a variety of green colors. Western Canada is the principal source of modern lapidary nephrite. Nephrite jade was used mostly in pre-1800 China as well as in New Zealand, the Pacific Coast and Atlantic Coasts of North America, Neolithic Europe, and southeast Asia.

The name nephrite is derived from lapis nephriticus, which in turn is derived from Greek λίθος νεφρίτίκος; νεφρός λίθος, which means 'kidney stone' and is the Latin and Greek version of the Spanish piedra de ijada (the origin of "jade" and "jadeite"). Accordingly, nephrite jade was once believed to be a cure for kidney stones.

Besides the terms already mentioned, nephrite has the following synonyms and varieties: aotea, axe-stone, B.C. jade, beilstein, kidney stone, lapis nephriticus, nephrit, nephrita, pounamu, New Zealand greenstone,New Zealand jade,spinach jade (dark grayish green), and talcum nephriticus. Tomb jade or grave jade are names given to ancient burial nephrite pieces that have a brown or chalky white texture as a surface treatment.