Germanium is opaque to visible light, but transparent to infrared (heat). That makes it useful for heat-sensing cameras where it can be used to block visible light and make the camera sensitive only to infrared.
Lenses made out of germanium can also focus infrared light, which is neat, but they are very expensive even on eBay. This is just a flat germanium window, such as you might use to cover a CCD for example.
Source: eBay seller ohoa13
Contributor: Theodore Gray
Acquired: 3 June, 2003