gems, jade, amphibole jade, nephrite, serpentine, archaic jade, China

In this study, serpentine archaic jades from the pre-Han Dynasty were mineralogically analyzed to determine their components. The primary minerals identified were antigorite, chrysotile, and lizardite, which often intergrow intimately, with antigorite being the most common. Secondary minerals such as chlorite, dolomite, magnesite, talc, and brucite were occasionally present in significant amounts. Accessory minerals, including magnetite, chromite, graphite, and limonite, appeared in minor quantities, typically as fine-grained inclusions or veinlets. Quartz and amorphous substances, especially limonite, were frequently found in the weathered or whitened portions of the jade. The color of serpentine archaic jades is closely related to their mineral content. Higher concentrations of the three main serpentine minerals typically result in greener jade. The jade's color is also influenced by the relative abundance of magnesium (Mg), iron (Fe), and chromium (Cr). Higher Fe and/or Cr contents yield greener jade, whereas higher Mg content leads to a more grayish-white appearance. Submajor minerals also affect the color: chlorite enhances greenness, while dolomite, magnesite, brucite, and talc intensify whiteness. Accessory minerals like magnetite, chromite, and graphite contribute to a darker gray hue and reduce transparency. Limonite, a weathering product, imparts a yellowish-brown color. Additionally, magnetite and chromite increase the hardness and specific gravity of the jade, whereas brucite, chlorite, and talc reduce hardness, with talc giving the jade a greasy or pearly luster. The crystal habit, microstructure, and texture of the jade also significantly influence its perceived quality. Long-term weathering after the jade's creation induces three major secondary changes in its mineral composition and texture. Weathering causes the surface to become mottled and pitted, decreasing luster and transparency. It also increases porosity, reducing bulk density and mechanical strength. The most notable change is the whitening of jade," where the surface and sometimes inner parts of the jade turn whitish. This process can be classified into three types: self-pulverization, recrystallization, and substitutional transformation. The "whitening of archaic jade" is not due to calcification or intentional heating to facilitate carving or achieve a white surface."