gems, corundum, properties.3, synthetic corundum.7, spinel, chrysoberyl, quartz, tourmaline, zircon, scapolite, spinel inclusions

W.F. Eppler examines the inclusions responsible for asterism in spinel and chatoyancy in chrysoberyl, quartz, tourmaline, zircon, and scapolite, emphasizing the need to distinguish solid crystals from apparently similar hollow channels and to assess whether inclusions were trapped during growth or formed subsequently by exsolution. In natural spinel, oriented rutile needles following octahedral edges provide one explanation for asterism, but other fibrous inclusions remain unidentified. Sphene crystals in Ceylon spinel illustrate the distinction between relatively large primary crystals enclosed during host growth and minute needles attributed to exsolution. Partial dissolution and the arrangement of larger crystals on former growth planes support their primary origin, whereas their sparse distribution prevents them from producing a star. Burmese spinels contain additional unidentified dark crystals and minute brownish particles tentatively interpreted as rutile; patterned concentrations around larger inclusions suggest localized strain may influence exsolution. Experiments with synthetic spinel show that heat-induced separation of excess alumina can generate oriented plates and weak asterism, but the resulting effects lack commercial value. Elongated gas inclusions can also produce a four-rayed star in suitably oriented cabochons. Examination of a sharply chatoyant chrysoberyl reveals short, birefringent needles with a refractive index higher than the host, contradicting an explanation based exclusively on hollow channels for that specimen. Their identity and origin remain uncertain, although exsolution is suggested. Similarly, inclusions in a rose-coloured tourmaline that resemble channels in reflected light prove in transmitted light to be brown, birefringent crystal fibres with a higher refractive index than tourmaline. Eppler interprets these as primary inclusions formed alongside the host. Quartz cat’s-eye and tiger’s-eye are discussed in terms of aligned amphibole-asbestos fibres, while an earlier observation of zircon chatoyancy is attributed to unidentified minute crystals parallel to its principal axis. Madagascar scapolite displays a combination of hollow and liquid-filled channels, crystalline needles, and oriented blades, all potentially contributing to its cat’s-eye effect. The blades are interpreted as products of exsolution within the marialite–meionite series. Photomicrographs demonstrate that similar optical phenomena can arise from inclusions differing in composition, physical state, morphology, and developmental history, making detailed optical examination essential to explaining individual specimens.