corundum, Australia.12, gems

Chalmers surveys the occurrence, mining history, geological setting, and gem quality of diamonds, opals, sapphires, rubies, spinels, and beryls in New South Wales, drawing on published accounts, museum specimens, and personal observations. The principal historical diamond fields are Cudgegong, Bingara, and Copeton, where diamonds occur in ancient river gravels, commonly beneath basalt, and in younger deposits derived from their erosion. Recovery from gold-dredging operations near Wellington represents a more recent development, although the material is predominantly industrial quality. The author reviews competing explanations for the diamonds’ primary origin, including early suggestions of formation within the gravels, derivation from metamorphosed sediments, and association with basic igneous rocks. Reports of diamonds in volcanic breccia and dolerite are discussed cautiously because subsequent discoveries have not supplied adequate confirmation. New South Wales diamonds are generally small but commonly of good quality, with yellow and other colors recorded; claims of exceptional hardness are qualified by the directional nature of diamond hardness and conflicting experimental observations. Opal occurrences are divided between relatively restricted deposits in igneous rocks and the economically important sedimentary fields. At Tintenbar, precious opal occupies cavities in decomposed basalt, but its tendency to crack curtailed mining. White Cliffs and Lightning Ridge occur in Lower Cretaceous sedimentary rocks, principally clayey sandstones associated with silicified marker beds known as bandstone. At White Cliffs, precious opal occurs within common opal along bedding planes and joints and replaces fossils, wood, and minerals, including the distinctive pseudomorphs called opal pineapples. Lightning Ridge, especially noted for black opal, produces isolated nodules known as nobbies. The widespread distribution of suitable sediments suggests possibilities for further discoveries. Sapphire is chiefly recovered from ancient or reworked gravels, particularly in the New England district. Stones include very dark blue, green, yellow, and parti-colored material; microscopic observations reveal minute vesicular inclusions, rutile, and color banding, although their diagnostic significance remains unproven. Opaque bronze-colored chatoyant corundum also offers possibilities for cutting black star sapphires. Chalmers questions many older ruby and spinel identifications, noting that pale-pink corundum is not true ruby and that some reported material was probably garnet or zircon, while small red spinel crystals have been confirmed by testing. Beryl occurs in pegmatites around Emmaville and Torrington, with transparent pale yellowish-green, deep-green, and golden material represented in museum collections. A small emerald industry near Emmaville operated intermittently between 1890 and 1909. These emeralds are comparatively pale, and the article records their filter reaction, relatively low specific gravity, and refractive index, placing the deposits’ commercial history alongside their mineralogical characteristics.