Moldavite

Moldavite is the tektite of the Central European strewn field. The impact that formed the Ries crater (about 16 miles, or 26 km, across) and the nearby Steinheim crater in southern Germany melted surface rock and splashed the melt to the northeast, where it fell as glass about 500 km away, mostly in southern Bohemia in the Czech Republic, with finds also in Austria and Germany. It is found today in Middle to Upper Miocene sediments as droplet-shaped pieces up to a few centimetres across, often deeply etched by acidic groundwater; much of the original strewn field has been eroded or buried. The name comes from the Moldau (Vltava) River.

Natural moldavite shows flow lines (swirls), abundant gas bubbles that are often elongated, and the diagnostic "wires" or curls of lechatelierite, a high-temperature silica glass that stands out under a loupe because of its lower refractive index; lechatelierite is not found in imitations. Imitations are usually green bottle glass or glass made in China, cast and then acid-etched (likely with hydrofluoric acid) to copy the natural surface. Compared with moldavite (RI about 1.49, density about 2.35, inert in UV), Chinese imitations studied by GIA had RI 1.520, density 2.52–2.53 and chalky short-wave UV fluorescence, though some fakes do not fluoresce; GIA also identified one by FTIR spectroscopy. Warning signs reported by GIA, Geology.com and the Moldavite Museum include few or no visible bubbles, unusually vivid green colour and shiny surface, identical "natural" shapes, faceted stones over about 2 carats, very large pieces (few real moldavites exceed 100 g) and sellers who cannot name the find locality.

Moldavite is brittle and about as hard as ordinary glass, so it is best set in earrings, pendants and pins that are not exposed to knocks or abrasion; in rings it should be protected by a metal bezel.

Moldavite has been cut into faceted and cabochon gems since the mid-1800s. GIA reports that faceted moldavite was popular in Czech jewellery in the second half of the 19th century, often combined with Czech garnets (pyrope) or small river pearls, and that imitations of green bottle glass followed; in a study of five 19th-century moldavite sets in the Museum of Decorative Arts in Prague, four contained only glass imitations.

Geology.com notes that moldavite's extraterrestrial-sounding origin appeals to people interested in New Age and astrological practices, and that many buyers believe it has healing properties and pay a premium for it; it adds that there is no scientific evidence of any medical effect beyond placebo.