that are shaped like octahedrons; crystals commonly cubes Opal White Curved, 5-6 conchoidal fracture; may appear glassy, greasy, resinous, or dull; milky white and bluish-white precious opal shows plays of colors D. Cannot be scratched by an ordinary pocket knife but can be scratched by quartz Feldspar White Glassy or pearly 6 luster; good cleavage in 2 directions that meet at an angle of 90° or near 90°; common in granite and pegmatite rocks Opal White Curved, 5-6 conchoidal fracture; may appear glassy, greasy, resinous, or dull; milky white and bluish-white precious opal shows plays of colors Quartz White Curved conchoidal 7 fracture; occurs as milky quartz, chert, and chalcedony; crystals commonly 6-sided prisms with pyramid-like