Promethean quartz is a crystalline material.

When a Gamma Quadrant lifeform enclosed in a crystal was brought to Deep Space 9 in 2369 by Vash, a Bajoran working for the assay office thought it to be Promethean quartz, but according to Vash the molecular density and refraction index were much higher than with normal Promethean quartz. (DS9: "Q-Less")