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Impact Glass: How Asteroid Strikes Turn Rock Into Tektites and Moldavite

By Digital Towns Crystals · Last reviewed October 8, 2026

Short answer: impact glass is target rock melted by a hypervelocity asteroid or comet strike and cooled too fast to crystallize. Shock pressures above about 60 GPa melt rock completely, and the melt can pass 1,700°C, hot enough to fuse quartz into lechatelierite. Droplets thrown hundreds of kilometres become tektites such as moldavite; melt that stays near the crater ends up in suevite or impact melt rock. The whole excavation is over in a minute or two.

What happens in the first two minutes of an impact?

No large impact has ever been watched by people, so the sequence below comes from crater studies, experiments and calculations. Tulane University's natural disasters notes on impacts put the entire excavation at 1 to 2 minutes.

  1. Arrival at cosmic speed. Small meteorites have struck Earth at 4 to 40 km per second, and large bodies lose little speed in the atmosphere. Sandatlas describes the Ries impactor in Germany as roughly 1,500 metres across, hitting at an estimated 20 km/s.
  2. The shock wave. Contact sends a compressional wave into the ground. According to the World of Earth Science entry on shock metamorphism, typical shock pressures run from 0.5 to 100 GPa, applied almost instantaneously. The entry contrasts this with the long-term contact and regional conditions behind ordinary metamorphic minerals.
  3. Melting. Above about 60 GPa rocks can melt completely. The melt gets far hotter than volcanic lava: lechatelierite, fused silica, forms above 1,700°C, and the same entry gives impact melt temperatures that may exceed 10,000°C, enough to liquefy limestone.
  4. Ejection. A jet of fragmented rock, dust and melt is expelled upward and outward. Some ejecta vaporizes.
  5. Flight and quench. Melt droplets fly through the atmosphere, take aerodynamic shapes and freeze before they can grow crystals. That is a tektite.
  6. Collapse. A rarefaction wave rebounds, uplifting the crater floor; ejecta settles into a blanket that is thick at the rim and thins outward.

Glass is the signature of steps 3 and 5. Crystals need time to organize, and impact melt is denied it.

Which kinds of impact glass are there?

Product Where it ends up How it cools Example
Tektites Strewn fields hundreds of kilometres away Quenched in flight Moldavite, australites, bediasites
Glass in suevite Crater fill and nearby ejecta Quenched as fragments in breccia Ries suevite
Impact melt rock Pools in the crater Slowly, may partly crystallize Mistastin, Labrador
Desert glass In and near the target area Quenched Libyan desert glass

The Planetary Science Institute defines suevite as an impact breccia of angular fragments of different rock types with glass inclusions in a fine-grained matrix, first recognized at the Ries. When so much melt forms that it pools in the crater and cools slowly, PSI calls the result an impact melt rock; at the Mistastin structure in Labrador it forms a sheet about 80 metres thick with columnar joints, like a lava flow.

The Ries shows how extreme the pressures were. Sandatlas reports that graphite in gneiss beneath the crater was turned into microscopic diamonds, up to 0.3 mm, by about 60 GPa, and that this diamond-bearing impact rock is widely used to build houses in the town of Nördlingen. Libyan desert glass is covered in its own guide; the debate there was whether a ground impact or an airburst did the melting.

Where are the tektite strewn fields?

PSI's tektite page names four main strewn fields, each with tektites of similar chemistry:

Strewn field Source crater Age Names in the trade
Central European Ries, Germany, about 24 km About 14.8 million years Moldavite
North American Chesapeake Bay, about 85 km About 35 million years Bediasite (Texas), georgiaite
Ivory Coast Bosumtwi, Ghana Not given by PSI Ivorite
Australasian Unknown About 0.7 million years, possibly younger Australite, indochinite, chinite

The ages and sizes come from different sources. Eos reports that argon-argon dating puts the Ries at about 14.8 million years, and that it may have formed together with the smaller Steinheim Basin in a double strike. The USGS fact sheet on the Chesapeake Bay crater describes a 3 to 5 km bolide that blasted a crater 85 km across and 1.3 km deep about 35 million years ago, now buried 300 to 500 metres beneath southern Chesapeake Bay. The Australasian field is the odd one out: PSI says it covers more than 10% of Earth's surface and still has no confirmed crater.

Most moldavite fell in what is now the Czech Republic, northeast of the Ries. Geology.com notes that the impacting bodies arrived from the southwest and splashed melt to the northeast, across parts of Germany, Austria and the Czech Republic. The moldavite guide covers the collecting history.

How can impact glass be told from volcanic glass?

Tektites and obsidian can look alike, and PSI warns that tektites can be mistaken for obsidian or pitchstone. The clearest difference is water.

  • Water. PSI states that tektites contain no, or almost no, water. The International Gem Society explains that a UV-vis-NIR spectrometer shows sharp water bands at 1380 and 2210 to 2250 nm in obsidian and other natural glasses but not in tektites.
  • Silica. IGS gives tektites as about 75% silica plus aluminum, iron, calcium, sodium, potassium, magnesium, titanium and manganese. Libyan desert glass is nearly pure silica, according to Curtin University.
  • Inclusions. Rounded or torpedo-shaped bubbles, swirl striae and, in some moldavite, wire-like lechatelierite. Geology.com describes lechatelierite as a mineraloid formed only by instantaneous events: asteroid impacts, atomic blasts and lightning strikes.
  • Numbers. IGS lists tektite hardness as 5 to 6.5 and refractive index 1.46 to 1.54, rising with iron content. Its table gives Libyan desert glass a specific gravity of 2.21 with RI 1.462; Geology.com gives moldavite 2.32 to 2.40.

What about shatter cones and coesite?

These are the field proofs of an impact rather than collectibles in their own right. PSI says shatter cones, cone-shaped fractures from under 1 cm to more than 5 m, are found only at impact structures and nuclear test sites, and they are the only shock indicator visible to the naked eye. Coesite and stishovite, high-pressure forms of quartz, settled the Ries debate: PSI credits their discovery by Eugene Shoemaker and Edward Chao in the 1960s with overturning a century of volcanic interpretation.

What do people get wrong about impact glass?

  • "Tektites are pieces of the meteorite." PSI describes them as droplets of molten target rock, that is, melted Earth rock, ejected and fallen back several hundred kilometres away.
  • "It is a crystal." Impact glass has no crystal lattice. That makes it a mineraloid, covered in the mineraloids guide.
  • "Any glass with bubbles is a tektite." Bubbles are common in factory glass too; flow structure, lechatelierite, near-zero water and plausible price per gram are what matter. The fake moldavite guide covers the checks.
  • "Fulgurites are impact glass." Lightning also makes lechatelierite, but a fulgurite is a fused tube of sand or soil, not ejecta.
  • "Tektites are a modern curiosity." PSI notes the first written reference was about 1,050 years ago, by Liu Sun in China, who called them "Inkstone of the Thundergod."

Buying impact glass by the gram

Impact glass is sold by weight, so compare price per gram across sizes and expect it to rise with size, clarity and surface sculpting. Ask for the strewn field or country, look for a natural, etched surface on rough pieces, and treat a perfect, bubble-free, evenly colored "tektite" at a bargain price with suspicion. Keep tektites away from hard stones in storage: at Mohs 5 to 6.5 they scratch.

  • Moldavite: rough Czech impact glass sold by weight, from 0.5 to 0.6 g pieces at the low end to 4.5 to 5 g pieces, a clear way to see how price per gram climbs with size.
  • Libyan Desert Glass: sold by the gram, from about 1.75 to 2 g up to a 13 to 13.5 g piece; the pale yellow, nearly pure silica glass makes a useful contrast with green moldavite.
  • Black Obsidian Crystal Sphere Polished Volcanic Glass: not impact glass, but the volcanic glass most often confused with dark tektites; worth owning as a reference.

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Frequently asked questions

What is the difference between a tektite and impact glass?

Impact glass is any glass made by an impact. Tektites are the subset thrown far from the crater, quenched in flight and found in strewn fields hundreds of kilometres away. Glass that stays near the crater ends up in suevite, impact melt rock or local melt sheets.

How hot does rock get in an impact?

Shock pressures above about 60 GPa can melt rock completely, and impact melt is far hotter than lava. Lechatelierite, found in some tektites, requires more than 1,700°C, and impact melt temperatures may exceed 10,000°C.

How did moldavite form?

About 14.8 million years ago an asteroid struck southern Germany and made the 24 km Ries crater. Melted surface rock was splashed northeast and fell as green glass across parts of Germany, Austria and the Czech Republic.

Are tektites from space?

No. They are droplets of terrestrial target rock melted by an object from space, thrown into the atmosphere and fallen back to the ground. They contain almost no water, unlike volcanic glass.

How do you tell a tektite from obsidian?

Obsidian contains water, which shows as absorption bands on a spectrometer; tektites contain almost none. Tektites also show swirl striae, rounded or torpedo-shaped bubbles and sometimes lechatelierite inclusions, and they come from known strewn fields.

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