What are impactites?
Impactites are rocks on Earth that have been impacted by asteroids (very large meteorites) or comets. Following such a meeting of worlds, terrestrial rock is shocked, shattered and melted. The most energetic impacts can even result in new minerals — including diamonds, says James Hyslop, head of Science and Natural History at Christie’s.
Impactites can be found on or beneath the floor of an impact crater, in the rim, or as ejecta (material ejected from the crater following the collision). Suspected impact craters are identified by the presence of impactites, as well as other impact products such as impact glass.
Was such an impact responsible for a German city reputedly being made of diamonds?
‘Yes. 15 million years ago there was a massive asteroid impact in what is now Western Bavaria. An impact crater 24km in diameter resulted, and the beautiful German town of Nördlingen now sits in the crater,’ says Hyslop.
As a result of the extraordinary amount of heat and pressure created by the impact, tens of thousands of tons of micro-diamonds formed in what was a fortuitously located graphite-rich deposit. The town was built with rock quarried from the site, and as a result, the stone buildings are filled with millions and millions of tiny diamonds.
‘Interestingly, the same event is responsible for Moldavite, a glass that splashed out of the crater and flew hundreds of miles to the Moldau River region in the Czech Republic,’ adds the specialist.
How is impact glass different from impactite?
Impact glass is a variety of impactite which reached a very high temperature then cooled very quickly, and whose silica concentration is sufficiently high that it is more of a glass than solidified molten rock.
‘The most famous impact glass is Libyan desert glass, which is found in the Sahara,’ says Hyslop.
Tell me more about Libyan desert glass.
‘Libyan desert glass comes from the border region of Egypt and Libya, where 29 million years ago an enormous asteroid impact occurred and the sands of the Sahara were literally melted into glass,’ says Hyslop.
To melt the sand, temperatures of more than 1600°C (2900°F) are required. Lava — the hottest home-grown material on Earth’s surface — is nowhere near that temperature.
‘This is not the only proof that an extraterrestrial bombardment was responsible for these forms: meteorite particles have been detected in specimens of Libyan desert glass,’ adds the specialist.
What do collectors look for?
Although a large amount of Libyan desert glass has been recovered, the vast majority of specimens are rather small, not terribly attractive, and opaque. ‘Most pieces are about the size of a thumbnail,’ says Hyslop. ‘Large, translucent examples are much more sought-after.’
The glass can range from off-white to a robust yellow, depending on the mineralisation of the material melted. Today, much of it is used for carvings, the more deeply hued material being especially prized by jewellers. ‘Given its popularity, the source is drying up, and it is becoming increasingly difficult to find,’ explains Hyslop.
The largest known specimen of Libyan desert glass, which is being offered at Christie’s until 23 February, weighs an impressive 26.4 kg (58 lbs). ‘It has a brilliant sculptural quality — it looks like Kryptonite,’ says the specialist.
‘Having a huge piece of the desert that was turned to glass by an asteroid impact millions years ago is a mind-blowing conversation piece,’ he adds.
Libyan desert glass has fascinated humans for thousands of years. It was used to make tools during the Late Pleistocene epoch.
In 1922, when the British archaeologist Howard Carter unsealed the tomb of the pharaoh Tutankhamun, he discovered a large piece of Libyan desert glass carved as a scarab set in a beautiful necklace (above), which had lain untouched for more than 3,000 years.
How has the impactite market evolved?
‘Impactites are very much a nascent market, but along with meteorites, they have recently seen a spike in prices with a lot of room still to grow,’ says Hyslop.
‘Each specimen has so many aspects that can beguile, so it is rare for two people to agree on their favourite. Prices are doubling or tripling the estimates we set. Last year, a piece of Libyan desert glass with an estimate of $800 ended up selling for $20,000.’