These butterscotch-colored wulfenite gemstones show their tetragonal crystal structure. Some gems in the tetragonal crystal system form as rectangular prisms with square terminations, like these. Others can take the shape of rectangular prisms with pyramidal terminations. Some form as dipyramids, two 4-sided pyramids joined at their bases. Wulfenites on a limonite matrix, San Juan Poniente Shaft, Ojuela Mine, Mapimi, Durango, Mexico. © Rob Lavinsky, www.iRocks.com. Used with permission.
“Amorphous” gems have no crystal structure.
Selected Minerals by Crystal System
Notes
1. Boleite has also been classed as a tetragonal, pseudo-cubic crystal.
2. Synthetic, lab-created material.
3. May lose their crystalline structure due to radiation (metamictization).
4. In modern times, hyacinth or jacinth usually refers to reddish brown zircon, which belongs to the tetragonal crystal system. However, historically it has also been used to refer to hessonite (cubic), topazes (orthorhombic), and various other gemstones.
5. Mimetite has also been classed as a monoclinic, pseudo-hexagonal crystal.
6. Both natural and synthetic stones. (Natural moissanite crystals are too small to cut).
7. Any beryl variety not aquamarine, emerald, goshenite, heliodor, morganite, or red.
8. Many of the materials mineralogists have classed as trigonal crystals have been classed by gemologists as hexagonal crystals in a trigonal subclass. Thus, you’ll find many of the gems listed here classed as hexagonal in their individual gem listings.
9. Wollastonite has monoclinic and triclinic polytypes.
Additional Crystal System Information
For more detailed information, consult the individual gem listings as well as these International Gem Society articles:
IGS Newsletter