corundum, gems

Farn presents a practical account of sapphire identification, concentrating on the use of absorption spectroscopy when microscopic examination reveals no diagnostic inclusions. Drawing on routine testing experience, he describes an initial assessment of colour, cutting, and general appearance, followed by examination with a microscope or hand lens. Rotating the stone permits observation of silk, crystals, laths, twin planes, liquid feathers, two-phase inclusions, and colour zoning, which he regards as evidence of natural origin and, in favourable cases, geographical provenance. Dichroism and doubling of rear facets provide additional optical information, while refractive-index measurements establish corundum when an apparently inclusion-free specimen requires further investigation. Surface chatter or shatter marks are considered supplementary clues associated particularly with cheaply cut synthetic stones, although Farn explicitly acknowledges that they are not conclusive. The principal discussion concerns an absorption feature near 450 nanometres, which he presents as diagnostic of natural blue sapphire and absent from synthetic sapphire. His procedure begins by adjusting the spectroscope slit against daylight until the Fraunhofer lines appear sharp. A microscope with its eyepiece removed serves as an optical support, and a temporary ground-glass screen assists alignment of the lamp, specimen, and transmitted beam before the spectroscope is placed over the microscope tube. This arrangement permits examination of sapphires while still mounted in jewellery. Farn emphasizes adequate illumination and exclusion of stray light, recommending an aperture in cardboard beneath small specimens so that only light transmitted through the stone enters the instrument. He describes variation in absorption strength, contrasting the pronounced complex around 450–460 nanometres in iron-rich Australian sapphires with the weaker spectra commonly encountered in Ceylon material. Turning the specimen and slightly tilting the spectroscope may improve visibility of faint features. Practice with known natural stones is recommended to establish familiarity with the position and appearance of the spectrum before modifying the apparatus. The article also addresses heat generated by powerful lamps, noting that concentrated illumination can damage gemstones. A water-filled glass flask is proposed as both a condenser and a heat filter, while an additional flask containing copper sulphate solution suppresses the red portion of the spectrum and may enhance weak blue-region absorption. The account combines an experience-based testing sequence with detailed advice on alignment, illumination, filtering, and observation, illustrating Farn’s reliance on spectroscopy as a rapid aid to sapphire identification.