corundum, gems

Farn outlines a systematic approach to ruby identification, emphasizing the distinction between establishing a stone as corundum and determining whether it is natural or synthetic. Although experienced observers may recognize characteristic appearances and inclusions immediately, he argues that laboratory conclusions require corroborating tests rather than intuition or commercial reputation. Refractive-index measurements and the characteristic chromium spectrum establish ruby but do not distinguish natural from synthetic material. The spectrum is described through its red doublet, broad absorption across the yellow-green region, and fine absorption lines in the blue; Farn favours spectroscopy for small or worn stones because it avoids contact with the comparatively soft refractometer prism. Microscopic examination provides the principal evidence of origin. Features discussed include rutile silk, mineral crystals, twin planes, feathers, and the uneven colour distribution termed “treacle,” which Farn associates particularly with Burmese rubies. He contrasts these with features observed in Siamese material, including distinctive halo-like feathers surrounding opaque inclusions, while acknowledging that commercial geographical labels may follow colour and appearance rather than actual provenance. Particular attention is given to misleading similarities: natural crystals may resemble gas bubbles, synthetic bubble groups may suggest natural feathers, and internal colour effects may resemble striae in glass. Comparison specimens and repeated observation are therefore recommended. For apparently inclusion-free stones, immersion in a liquid approaching ruby’s refractive index reduces surface interference and assists the search for bubbles or curved growth structure. Farn describes rotating the specimen while adjusting focus, condenser height, diaphragm aperture, and illumination angle to reveal faint internal features. Excessive magnification may obscure slight curvature by restricting the visible length of a growth line, while focusing beneath the specimen can mistakenly bring bubbles or striae in the glass immersion cell into view. When microscopy remains inconclusive, he discusses X-ray transparency, X-ray-excited fluorescence and phosphorescence, and immersion contact photography. He regards phosphorescence following X-ray excitation as strong evidence of synthetic origin, illustrating the need for continued observation with a specimen whose afterglow appeared only after a delay. Short-wave ultraviolet fluorescence is treated more cautiously: differences in brightness may assist comparison within a parcel, but variation among natural rubies prevents its use as an independent diagnostic test. Immersion contact photography can disclose curved structure that escapes direct microscopic observation, and simple low-power examination against a diffusely illuminated white background may also prove effective. Throughout, Farn stresses the difficulty created by the closely similar optical and chemical properties of natural and synthetic ruby. His central conclusion is that careful interpretation of internal structure and inclusions, supported by complementary tests where necessary, provides the strongest basis for identification.