Rock crystal

Rock crystal was another name for colorless, transparent quartz.

Description
This clear semi-precious stone was considered lower grade if it was soft or brittle, but was frequently used to adorn furniture or decorative head gear. High grade samples free from impurities were used for optics, eyeglasses, magnifying glasses, and prisms. A typical specimen had a base value of 50 gp.

Powers
When a spell required an unspecified gem of a certain value, rock crystals were generally used. There was a process that could turn a quantity of rock crystals into a molten glass-like state that allowed it to be worked into weapons and other implements. When it cooled, the rock crystal was hard enough to be used for mace heads, spear heads, ram heads, or even climbing spikes. The process required three spells; two of which were minor but kept secret, and Veladar's vambrace.