Does anyone know a crystal with refractive index of water?

[MaxL]

When I was a little kid, I met a gemologist who showed me a cool trick with a ring he'd bought for his wife. When he submerged it in water, the crystal disappeared!

As I now know, this crystal had an index of refraction of 1.33.

Anyone know what this crystal is and/or how I can get my hands on one?

 

[hamster143]

Cryolite?

http://en.wikipedia.org/wiki/Cryolite

http://cgi.ebay.com/FREAKINGCAT---2.46-ct---CRYOLITE---GREENLAND_W0QQitemZ120499866185QQcmdZViewItemQQimsxZ20091205?IMSfp=TL091205011007r1225