  Very large Congo cube diamond.
This was a huge, HUGE 7.45ct diamond before it exploded. "Congo Cube" diamonds are clusters of small yellowish crystals. They are worth far less than clear gem-quality diamonds, but one this big is still several hundred dollars.
When I say "exploded" I mean it seriously exploded, in fact with such force that two diamond fragments dug holes into the lens we were using to photograph the event. It wasn't supposed to explode. I was heating it with an oxy-hydrogen torch to get it to catch fire and burn (for my Popular Science article about how to burn diamonds) when suddenly and without warning there was a tremendous SNAP, I felt something hit my hand hard, and we all simultaneously realized that this was a very expensive SNAP.
There must have been some kind of internal fracture that gave way as it reached yellow heat. We had successfully burned a smaller Congo Cube diamond and a low-grade regular diamond without incident, but this one was much bigger and thus more susceptible to uneven heating, I suppose.
Fortunately we were able to gather a few of the fragments and use them in subsequent photography runs, including showing how they burn in liquid oxygen.
Source: eBay seller sherl111
Contributor: Theodore Gray
Acquired: 28 June, 2009
Text Updated: 29 June, 2009
Price: $280
Size: 0.3"
Purity: >98%
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