Recently I posted on the origin of the English word crystal. In its earliest forms, it refers to petrified ice. Rock crystal, i.e. quartz, was used to make most of the earliest lenses. All evidence seems to show that pre-Medieval lenses were only used for starting fires and the like, not for looking at small things and making them look larger. Since the lenses made for starting fires were made out of what was considered petrified ice, it seems only appropriate that you can entirely skip the bit where the ice petrifies, and just make a lens directly out of ice for your fire-starting needs.
At least, this is true as long as you have a good quality water supply. This might not work if you live in Belgium.