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These are very beautiful, very fancy, and normally very, very expensive germanium lenses used to focus infrared light. They are completely opaque to visible light, but transmit heat (infrared light) easily. I bought two on eBay and then, as is my practice, asked the seller if they had any more. He turned out to be very nice and sold me 8 more lenses plus threw in a couple of really nice slabs (see below). Most of them have a chip or two around the edges, which may render them less than useful, and certainly reduce their value a great deal, but for my purposes the chips are if anything a good thing, since they let you see the unique fracture surface (similar to broken silicon). Without the chips these would just be polished blocks of something hard to identify.
They were described as having a thorium coating, which if it's there must be very thin since they register only barely above background radiation levels. Thorium fluoride is used as an anti-reflective coating on infrared lenses, so it wouldn't be surprising if these had such a coating, and it would be very thin.
Source: eBay seller 142pimp
Contributor: Theodore Gray
Acquired: 16 November, 2003