GEMSTONE FACETING ANGLES

Suggested Faceting Angles for Various Materials

CA= Critical Angle; RI= Refractive Index
Angles listed as: Crown Mains/Pavilion Mains Courtesy of the Facet Shoppe- John and Barbara Franke

MATERIAL	RI	CA	Vargas	M.D.R.	Schlagel

Almandine*	1.81	34.5	39/41	37/42	36/40
Andalusite	1.64	37.5	41/42	43/39	35/39
Apatite	1.64	37.5	41/42	43/39	35/39
Beryl	1.58	39.5	42/43	42/43	37/41
Chrysoberyl	1.75	35	40/42	37/42	36/40
Corundum	1.76	34.5	40/42	37/42	36/40
Feldspars	1.55	41	42/44	42/43	39/43
Fluorite	1.43	44	43/45	41/45	41/45
Grossular*	1.74	35	40/41	37/42	36/40
Iolite	1.54	40	42/43	42/43	38/42
Opal	1.45	43.5	43/44	41/45	41/45
Peridot	1.67	37	41/42	43/39	35/39
Pyrope*	1.73	35	40/41	37/42	35/39
Quartz	1.55	40	42/43	42/43	38/42
Rhodolite*	1.76	34.5	40/41	37/42	36/40
Scapolite	1.57	40	42/43	42/43	38/42
Spessartine*	1.80	33.5	39/41	37/42	36/40
Sphalerite	2.37	25	35/40	35/41	38/42
Sphene	1.89	35.5	38/41	35/41	37/41
Spinel	1.72	35.5	40/42	37/42	35/39
Topaz	1.63	38	41/42	43/39	36/40
Tourmaline	1.64	38	41/42	43/39	36/40
Zircon	1.94	31	37/41	35/41	36/40
Zoisite	1.70	35	40/42	43/39	35/39

* = Garnet Group

Glenn & Martha Vargas, "Faceting for Amateurs"

M.D.R. Manufacturing, "The Book of Gem Cuts"

Charles Schlagel, "Gem & Minerals" magazine, March 1966.

RI and CA are sometimes variable; these figures represent averages/approximations.
NOTES: Feldspars include Oligoclase-moonstone, Labradorite-Spectrolite, Orthoclase-sunstone, etc. The Quartz Group includes: Amethyst, Citrine, rose quartz, smoky quartz. Beryl includes aquamarine, emerald, morganite, goshenite, and heliodor. Tanzanite is a variety of Zoisite.
Remember the Critical Angle (CA) is what you want to be aware of when cutting the pavilion. Cutting pavilion facets at an angle less than the critical angle can cause your gem to "fisheye" or dump light rays out of the pavilion instead of reflecting them back through the crown. The Refractive Index (RI) is a ratio of the speed of light within a substance compared with the speed of light in air. It is pretty technical but the higher the RI the more sparkle, basically. There are many other terms and conditions that effect the color and sparkle of your finished gem and you may find it worth your while to buy a comprehensive reference or borrow one from the local library to learn these things. You can learn from your own experiments- discover the angles *you* like the best for each material!

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