Having had PM about refractive indexes, I figured I might as well post the answer here.
There's several lens materials that can be used nowadays:
plastic / CR39 - index 1.49
(regular) glass/ Crown glass - index 1.52
polycarbon / plutonite - 1.586 (commonly refered to as 1.6)
high index plastics - index 1.64 to 1.74
high index glass - up to index 1.9
Roughly you can say that with each 'step' in index, the lens will be 10% thinner. So, Crown glass is 10% thinner than CR39. Plycarbon is 10% thinner than Crown glass. Etc.
More about refractive indexes etc:
http://en.wikipedia.org/wiki/Corrective_lens#Refractive_index