Jadeite and nephrite differ in both chemical composition and crystalline structure. Jadeite is a silicate of sodium and aluminum and is classed as a pyroxene. Nephrite is a silicate of calcium and magnesium belonging to the amphibole group of minerals and is properly regarded as tremolite. In both types, the microscopic crystals are tightly interlocked to form a compact aggregate. Both jadestone types may be white or colourless, but colours such as red, green, violet, and gray may occur owing to the presence of iron, chromium, or manganese impurities, respectively. The most highly prized variety is jadeite of an emerald-green hue.