A famous precious gem found in China and called tianhuang has been investigated by such means as X-ray diffraction, infrared spectroscopy, differential thermal analysis and electron microscopy. The results indicate that this mineral is a rare sort of nacrite and can be grouped into three kinds according to gemmology:1. Tianhuang gel: having nacrite as main composition, occasionally with a small amount of illite.2. Tianhuang stone., a mixture of nacrite and dickite in varying proportions.3. Gold coated with silver: golden-yellow nacrite wrapped in dickite which is pure white in colour and transparent to semitransparent.This type of gem is very rarely seen in the world and is only found in Bantian gravel bed in Shoushan, Fujian province. It is so valuable that its price is three times as high as that of gold. The indicators for primary tianhuang ore are also discussed in this paper.