masses and as rhomb-shaped crystals; dilute hydrochloric acid may fizz slightly on dolomite Fluorite White Cleavage in 4 4 directions can give fragments that are shaped like octahedrons; crystals commonly cubes Opal White Curved, 5-6 conchoidal fracture; transparent _hyalite_ variety resembles ice D. Cannot be scratched by an ordinary pocket knife but can be scratched by quartz Opal White Curved, 5-6 conchoidal fracture; transparent _hyalite_ variety resembles ice Quartz White Curved, 7 conchoidal fracture; _rock crystal_ quartz commonly 6-sided prism with pyramid-like ends E. Cannot be scratched by quartz Topaz White Perfect basal 8 cleavage gives flat, plate-like fragments; notice hardness How To Use the Rock Identification Charts In the rock identification charts on pages 40-41, the Texas rocks described in this book are arranged in four major groups according to their texture.