Nephrite

Nephrite is a variety of actinolite-tremolite composed of densely interlocking microscopic fibers. This interlocking structure gives nephrite its extraordinary toughness, making it the toughest natural material known (tougher even than steel in terms of resistance to breaking).

It forms through metasomatic alteration of serpentinite or dolomitic marble. When silica-rich fluids interact with magnesium-rich ultramafic rocks or carbonate rocks, the resulting chemical reactions produce amphibole minerals that grow as tightly interwoven fibers. Iron content determines color: more iron produces darker greens and blacks, while iron-poor nephrite can be nearly white.

The white 'mutton fat' nephrite from Hotan, China has been prized for over 7,000 years and remains among the most valuable jade varieties. In Chinese culture, nephrite was jade for millennia before jadeite (the other jade mineral) was imported from Myanmar in the 18th century.

Nephrite is identified by its waxy to greasy luster, exceptional toughness (it's very difficult to break), and characteristic colors. Under magnification, the fibrous structure is visible. Specific gravity of 2.90-3.03 is lower than jadeite (3.30-3.38).

Distinguish from jadeite (higher SG, more vitreous luster, often more translucent), serpentine (softer at 3-5, lower SG), and aventurine (sparkly inclusions, quartz family). A simple test: nephrite is so tough that it's virtually impossible to break by hand.