An ornamental stone, jade is a name applied to two related minerals, the nephrite form of actinolite (a mineral that also includes a form of asbestos), and a mineral called jadeite. The former is a calcium magnesium iron silicate hydroxide, with the formula Ca2(Mg, Fe)5Si8O22(OH)2, and the latter is a sodium aluminum silicate, with the formula NaAlSi2O6. The two are closely related, and it wasn't until 1863 that people could distinguish between the two forms of this stone.
It is an exceptionally tough material, and was first used for things such as axe heads, knives, and weapons. Later, as other materials could replace jade as a weapons material, it became appreciated for its beauty.
Nephrite can be found in a creamy white form as well as a green color, while jadeite shows more color variation. Of the two, jadeite is rarer, and is the form of jade mostly used in Central America. Nephrite jade was used mostly in China.
The key known sources of nephrite jade in China for utilitarian and ceremonial jade items were during Neolithic times the now depleted deposits in the Ningshao area in the Yang Ze River delta (Liangzhu Jade culture 3400–2250 BC), in an area of the Liaoning province in Inner Mongolia (Hongshan Culture 3500–2200 BC) and, from about the earliest Chinese Dynasties until present, the jade deposits in most Western Chinese province of Xinjiang. There white and greenish nephrite jade is found in small quarries and as pebbles and boulders in the rivers flowing from the Kuen-Lun mountain range northward into the Takla-Makan desert area. River jade collection was concentrated in the Yarkand, the White Jade (Yurungkash) and Black Jade (Karakash) Rivers. From the Kingdom of Khotan, on the southern leg of the Silk Road, yearly tribute payments consisting of the most precious white jade where made to the Chinese Imperial court and there transformed into objets d'art by skilled artisans as jade was considered more valuable than gold or silver.
In New Zealand, where it is known as greenstone or pounamu, nephrite was fashioned for centuries by Maori to make weapons and ornaments, and is still widely used to make carved jewellery although the mining of it is restricted and closely monitored.