jade, jadeite, fei cui, California, gems

R.G. Coleman describes the geological occurrence, mineralogical properties, and ornamental potential of jadeite from the Clear Creek region of San Benito County, California. After distinguishing jadeite from nephrite and other green minerals marketed as jade, he reviews discoveries of jadeite-bearing boulders and bedrock exposures and reports additional occurrences, including a large isolated mass near Santa Rita Peak. The deposits lie within an elongated serpentine body forming a dome bordered by sedimentary rocks. Jadeite occurs principally in two settings: zoned, lenslike bodies enclosed by serpentine, and veins cutting schist masses within it. The lenses contain outer zones rich in prehnite, hydrogarnet, and sphene, an intermediate zone dominated by thomsonite, and a central mass of fractured green jadeite subsequently healed by white jadeite. Veins occur mainly near the margins of albite-acmite schist bodies, generally measure less than an inch across, and locally widen to eight inches. Larger veins likewise preserve an older green generation cut by younger white jadeite, whereas smaller white veins show relatively little subsequent deformation. Chemical analysis, optical measurements, density determination, and X-ray powder diffraction establish that the white material closely approaches pure jadeite. Its density is approximately 3.43, and its principal refractive indices are about 1.654, 1.657, and 1.666. The associated green jadeite contains appreciable diopside and acmite components; Coleman attributes its colour principally to iron incorporated through the acmite component. Despite the mineralogical purity of some material, the deposits had not yielded the emerald-green colour characteristic of imperial jade or abundant large masses of uniform appearance. Two-stage crystallization commonly produces contrasting colours and textures, with fine-grained green jadeite interrupted by coarser white material. Nevertheless, selected small pieces can produce attractive cabochons, and the green variety polishes more readily than the white. Variegated and brecciated boulders reaching three feet across offer possibilities for polished slabs. Coleman concludes that the locality has value as a source of lapidary material and potentially small ornamental pieces, while emphasizing the scarcity of uniformly high-quality masses, the difficulty of quarrying exceptionally tough rock, and the untested possibility that further exploration at depth might reveal better material.