Nephrite
Nephrite and jadeite both are mineral species arising from special metamorphic conditions. Seldom are these jades found in place. They usually occur as pebbles and boulders in present or ancient stream beds to which they have been carried from their original deposits by strong erosional forces. Nephrite is very often associated with other iron-bearing metamorphic rocks, such as hornblende schists and gneisses, and with its look-alike, serpentine. Jadeite, on the other hand, occurs with a variable suite of minerals that require high pressures and temperatures for their formation. Usually these minerals associated with jadeite include albite and quartz. As with nephrite, serpentine is often present. Jadeite and nephrite are almost never found in proximity with each other, attesting to the different sets of natural conditions under which they are formed.
Nephrite is a calcium magnesium silicate. It is a member of what is called the tremolite-actinolite series of minerals. Tremolite is a pure, white, calcium magnesium silicate, and actinolite is a green calcium magnesium iron silicate. There are any number of mineral samples whose composition falls somewhere between the two, nephrite among them. The tremolite-actinolite series itself is part of a larger family of mineral species known as amphiboles. Jadeite is not even closely enough related to nephrite to be part of the amphibole group. Nephrite differs from the other tremolite-actinolite minerals in that the typical fibrous crystals of the group are very compactly and tightly matted and meshed together. This produces an extremely tough, tufted structure. Check other diamond jewelry
Its hardness is only 6y2, which is not extreme among minerals. However, its toughness, due to the gross structure, is so great that very thin and delicate objects carved from it are not nearly so fragile as they look. It was an ideal material for primitive tools, which in some ways were more durable than the iron implements introduced later in Chinese culture. Nephrite is three times as dense as water, but its specific gravity, due to compositional and impurity differences among selected samples, varies from 2.90 to 3.01. This value, unfortunately, is too close to the density of its most troublesome substitutes, but serves admirably to distinguish it from jadeite. Sometimes nephrite is green. Certainly its best-known and more highly prized pieces are green. Shades of white, brown, yellow, and gray-to-black are just as natural to it. Even blue nephrite is known. Almost all these colors, as suggested earlier, are due to the presence of iron in its composition. Oddly enough, white nephrite has been found with considerable iron content, so that at least part of the reason for coloring in this kind of jade is still a mystery.