Hinthar-taung, Wet-loo, Kyauk-Pyat-That, Mogok Township, Pyin-Oo-Lwin District, Mandalay Region, Myanmar
An exceptionally large crystal of painite-- crystals 2 cm are hard enough to find, this thing is a relative monster at 4.0.
Up until several years ago, this rare borate was touted as the world's rarest gemstone, with fewer than 20 carefully documented crystals having been collected since its discovery in the 1950's. Then in late 2005, the source was discovered, and they were finally made available to collectors.
It may look like a chocolate bar that's been lost in a sofa for years, but it's a good rock.