corundum, chemistry.2, inclusions.5, geology.11, Australia.12, gems

R.R. Coenraads examines the surface morphology of natural rubies and sapphires from volcanic-associated placer deposits in eastern Australia and Thailand, distinguishing features inherited from crystal growth and magmatic transport from damage acquired in alluvial environments. Samples from the New England gemfields and the Bo Phloi and Chanthaburi–Trat districts were studied with binocular microscopy, followed by scanning electron microscopy of selected cleaned and gold-coated specimens. The observed features fall into three broad categories: negative crystal impressions, resorption and etching structures, and fractures or impact damage associated with subsequent transport. Deep, sharply bounded geometric sockets are interpreted as impressions left by neighbouring minerals in a coarse crystalline aggregate containing corundum, anorthoclase, zircon, and spinel. Coenraads proposes that corundum-bearing assemblages formed from evolved, silica-poor magmas and were subsequently disaggregated and partly dissolved by alkaline basaltic carrier magmas during ascent. Most rounded surfaces are attributed to this magmatic resorption rather than prolonged stream wear. Their sculpturing reflects corundum’s trigonal symmetry and varies with surface orientation relative to the c-axis. Features include triangular prismatic hillocks, pyramids, inverted pyramidal pits, and shallow triangular or hexagonal depressions containing central remnants. Surfaces approximately parallel to the c-axis may display inclined stacks resembling elongated triangular bricks, while intermediate orientations favour triangular pyramidal depressions. Continued dissolution enlarges and merges these structures into irregular channels, porous textures, and deeply sculptured surfaces. Fine corrosion pits and microscopic steps indicate superimposed etching and successive layer removal. Curved or subparallel chatter marks are interpreted as products of mechanical stress; crystallographically controlled etching within them suggests that they formed before or during magmatic ascent and were subsequently modified by dissolution. Later mechanical damage includes conchoidal fractures radiating from impact points, chipped edges, and broken hillocks exposing clean fracture surfaces. Breakage along previously healed or partly healed fractures can reveal fingerprint-like networks of fine tubes, connecting external surface observations with familiar internal inclusions. Despite these examples of damage, many delicate surface projections remain intact. Their preservation, considered alongside spatial differences in corundum characteristics within individual placers, indicates limited downstream reworking and mixing. Coenraads concludes that the sources are likely to lie close to the placer deposits. The study demonstrates how successive generations of surface features can preserve evidence of crystal association, reaction with carrier magma, stress, and fluvial history, allowing rounded gem corundum to be interpreted through processes more specific than abrasion alone.