The Hard Science of Moldavite
Moldavite is a tektite: natural glass formed when an asteroid impact melted Earth's surface rocks. It is terrestrial material, not a piece of the asteroid.
How it formed
- About 14.8 million years ago (Miocene epoch) an asteroid, likely several hundred metres across, hit southern Germany and formed the Ries crater.
- Surface sands and sediments were instantly melted at temperatures well above 1,600 °C.
- The melt was ejected at high speed toward the east-northeast, travelling 200–450 km in a few minutes.
- Droplets and splashes cooled in flight into glass and fell across today's Czech Republic, Austria and eastern Germany: the Moldavite strewn field.
- Over millions of years, buried in sediments, chemical corrosion etched the surfaces into the sculpted, "hedgehog" textures collectors prize.
The nearby, smaller Steinheim crater (~3.8 km) has been proposed as a twin impact from the same event. Whether the two are exactly the same age is still debated.
Physical properties
| Property | Typical value | Why it matters |
|---|---|---|
| Material | Amorphous silica-rich glass | No crystal structure, so it is not a "crystal" in the mineral sense |
| Color | Bottle green, olive, forest green, brownish green | Caused mainly by iron; vivid emerald or lime green is suspicious |
| Hardness | About 5.5–7 Mohs | Scratches more easily than quartz; facets wear with daily use |
| Specific gravity | About 2.32–2.38 | A density test is one of the best home checks; many fake glasses are heavier |
| Refractive index | About 1.48–1.51 | Lower than most man-made glass |
| Water content | Extremely low (hundredths of a percent) | Formed in extreme heat; much drier than volcanic glass |
| Optical | Isotropic, often with strain | Swirls (schlieren) visible in transmitted light |
Chemistry
| Component | Approx. weight % |
|---|---|
| SiO₂ (silica) | 75–81 |
| Al₂O₃ (alumina) | 9–12 |
| K₂O (potassium oxide) | 2–3.5 |
| CaO | 1.5–3 |
| FeO (total iron) | 1–2 |
| MgO | 1–2 |
| Na₂O | under 0.5 |
Values are typical ranges from the literature and vary by locality. Low sodium and high potassium help distinguish Moldavite from bottle glass.
Inclusions: the fingerprints of an impact
- Lechatelierite: worm-like threads of nearly pure silica glass, formed only at extreme temperatures (lightning or impacts). Their presence is strong evidence of genuine Moldavite.
- Bubbles: mostly irregular, elongated or distorted, sometimes partially collapsed. Perfectly round bubble trains suggest man-made glass.
- Flow structures: swirled schlieren where melts of slightly different composition mixed.
Tektite, not meteorite
Meteorites are fragments of asteroids that survive the fall. Tektites are melted Earth rock. Moldavite's chemistry matches the sediments around the Ries crater, which is why scientists link the two. Other tektite fields include the Australasian (indochinites, australites), Ivory Coast and North American fields; Moldavite is the only green, gem-quality one.
Key evidence for the Ries link
- Radiometric ages of Moldavite and Ries impact glass match (~14.8 million years).
- Chemical and isotopic signatures match the Ries-area surface sediments.
- The strewn field fans out east-northeast of the crater, consistent with a single ejection direction.