Jade, or yu, as it is called in China, is a generic term for two different gems; nephrite and jadeite. It is thought the name comes from the Spanish 'piedra de ijada', loin-stone, jade having been recognised by the Amerindians as a cure for kidney ailments. Because of its beneficial effect on the kidneys, the stone was also known as 'lapis nephriticus'. That, as you might guess, is where the term 'nephrite' came from.
Jadeite and nephrite are both regarded in China as 'zhen yu', 'genuine jade'. It was not until the beginning of the 19th century that mineralogists realised that they were dealing with two distinct minerals, since they bear a considerable resemblance to each other in terms of their appearance, their hardness and the properties they exhibit when being processed. Both are tough, since they consist of dense, close-grained, matted aggregates, but they differ from one another in their chemical composition and colours. Nephrite ranges mainly from mid to dark green or grey-green, but it can also be white, yellowish or reddish. Rarer (more expensive), and somewhat tougher, jadeite displays hues which include green, but also white or pink, and reds, blacks, browns and violets. In both minerals, the way the colour is distributed varies a great deal. Only in the very finest jade is the colour evenly distributed. Both nephrite and jadeite often have veins, blemishes and streaks running through them, though these may not always be regarded as flaws. On the contrary, some of these patterns are considered particularly valuable.