gems, tektite, natural glass

This review introduces tektites as silica-rich natural glasses and examines their physical properties, historical uses, geographical distribution, characteristic shapes, and disputed origins. Tektites are described as containing approximately 68–82% silica and exceptionally little water, with an average water content near 0.005%. Reported hardness is 6–7, specific gravity 2.33–2.55, and refractive index 1.475–1.53; the latter two properties decrease as silica content increases. Colours are principally black or dark green, with brown and grey examples less common. The historical discussion traces their use as tools, amulets, and jewellery and reviews changing explanations ranging from artificial glass and volcanic ejecta to lunar or meteorite-related sources. Chemical differences from manufactured glass and obsidian, low water content, sparse bubbles, and distinctive surface features are presented as important evidence in these debates. Four major strewn fields are surveyed: Australasian, Ivory Coast, Central European, and North American. Their distribution is interpreted as recording discrete events rather than unrelated local formation, and possible associations with the Bosumtwi and Ries impact craters are discussed. Morphological categories comprise microtektites, commonly recovered from ocean-bottom cores; blocky, layered Muong Nong material; splash forms resembling spheres, droplets, dumbbells, and discs; and Australian flanged buttons. Surface pits, grooves, flanges, and spalled areas are considered in relation to aerodynamic shaping, atmospheric ablation, and thermal shock. Internal flow structures, compositional variations, and strain birefringence provide further evidence concerning melting and cooling. Particular attention is given to layered Muong Nong tektites and their comparatively high volatile contents, which may preserve information about precursor materials and proximity to deposition sites. The author reviews evidence favouring an impact-related terrestrial origin, including similarities to continental materials, differences from analysed lunar rocks, limited cosmic-ray exposure, and possible meteoritic metal inclusions. A proposed sequence involves impact melting of terrestrial rock or soil, ejection above the atmosphere, volatile loss and solidification, followed by atmospheric re-entry and surface modification. Competing interpretations and unresolved problems are retained within the discussion rather than presented as settled. A concluding aside treats claims of moldavite’s supposed mystical properties with scepticism and notes their connection with jewellery marketing.