“Cut Epidote,” Madagascar, 1.86 cts, by Didier Descouens. Licensed under CC By-SA 4.0.
The epidote mineral supergroup contains many related species of interest to collectors. However, epidote itself is the one most likely to be faceted into beautiful, albeit small and dark, gemstones.
Highest values go to transparency, color, and size.
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
| Faceted | All sizes |
| to /ct |
| Data | Value |
|---|---|
| Name | Epidote |
| Crystallography | All are monoclinic. Crystals prismatic and tabular; also granular, massive, fibrous; sometimes twinned, often striated. |
| Refractive Index | Varies by group member. 1.640-1.830. See "Identifying Characteristics" below. |
| Colors | Epidote shows shades of green, yellow, gray, grayish white, greenish black, black; usually very dark. Clinozoisite is colorless, pale yellow, gray, green, pink; often zoned. Piedmontite is reddish brown, black, rose red, pink. Allanite is light brown to black. Hancockite is brownish or black. |
| Luster | Vitreous; pearly on cleavages; massive materials dull. Allanite resinous, pitchy. |
| Polish Luster | Vitreous to greasy, allanite resinous. |
| Fracture Luster | Vitreous, pearly, dull. |
| Hardness | 6 - 7. Epidote sometimes slightly harder, piedmontite softer. |
| Wearability | Good to Poor |
| Fracture | Conchoidal to uneven |
| Specific Gravity | 3.21-4.20. See "Identifying Characteristics" below. |
| Birefringence | Varies by group member. 0.005-0.073. See "Identifying Characteristics" below. |
| Cleavage | Perfect 1 direction in all |
| Dispersion | 0.030 |
| Heat Sensitivity | Yes |
| Luminescence | Usually none. |
| Luminescence Present | No |
| Transparency | Transparent to Opaque. |
| UV Long | Usually inert, not diagnostic |
| UV Short | Usually inert, not diagnostic |
| Absorption Spectrum | Most members of the group have non-diagnostic spectra. Epidote has a very strong line at 455 nm. Sometimes, a weak line is seen at 475 nm. This spectrum is very sensitive to direction within the material and isn't visible in certain orientations. |
| Phenomena | Chatoyancy, very rare. (Epidote and clinozoisite). |
| Formula |
The general formula of the epidote group is X2Y3Z3(O,OH,F)13 where X = Ca, Ce, La, Y, Th, Fe, Mn; Y = Al, Fe, Mn, Ti; Z=Si, Be.
|
| Pleochroism |
|
| Optics | Biaxial. See “Identifying Characteristics” below. |
| Optic Sign | Biaxial +, Biaxial - |
| Etymology | Epidote is derived from the Greek for “increase,” since the base of the prism has one side longer than the other. Clinozoisite is the monoclinic dimorph of zoisite. Piedmontite (or piemontite) is named after the locality in Italy, Piemonte (Piedmont). Unakite is named after the Unaka range of mountains in the United States. Allanite is named after mineralogist T. Allan. Tawmawite is named after the Myanmar locality. Mukhinite is named after A. S. Mukhin, Soviet geologist. |
| Occurrence | The minerals of the epidote group form at low temperatures in low to medium-grade metamorphic rocks. Allanite is more commonly found in igneous rocks such as pegmatites. Clinozoisite and epidote are also found in igneous rocks, and piedmontite is found in schists and manganese ore deposits. |
| Inclusions | Fibrous inclusions, needles, growth zoning. |
Epidote: Baja California, Mexico (1.0), Kenya (1.2). Photo ? Joel E. Arem, PhD, FGA. Used with permission.
Although epidotes have loaned their name to the epidote mineral supergroup, the gems themselves form a clinozoisite-epidote solid-solution series within this taxon. In 2006, this grouping lost all varieties of zoisite, including its most well-known member, tanzanite. Zoisite is a orthorhombic polymorph of monoclinic clinozoisite. Mineralogists reclassified and removed zoisite from the group, since all other epidote members had monoclinic crystal systems.
Nevertheless, the supergroup still includes some interesting gemstones, both common and rare. It features species suitable for faceting and cabbing as well as beautiful crystals for collecting. These materials often contain fibrous inclusions. On rare occasions, they create a chatoyant “cat’s eye” effect in cabbed epidotes and clinozoisites.
Usually so dark in color, large faceted epidotes result in nearly black, lifeless gems. However, small stones, under 3 to 4 carats, can often turn out as bright and lively faceted gems.
Pistazite or pistacite is a trade name for yellowish green “pistachio-colored” epidote.
These gems would actually make better looking faceted pieces than epidotes. Although clinozoisites aren’t rare, they occur rarely in sizes over 5 carats. The best known source of gem material is Baja California, Mexico. Other localities occasionally yield fine gems. Clinozoisite rarely occurs in pure form. It usually contains some iron, like its series brother, epidote. Clinozoisite shares a chemical formula and outward appearance (but not a crystal system) with zoisite.
Clinozoisite: Mexico (1.18). Photo ? Joel E. Arem, PhD, FGA. Used with permission.
Very dark in color and seldom cut, allanite contains rare-earth and radioactive elements. As a result, these gems become metamict. In other words, they have severe damage to their internal crystalline structure.
This New Jersey gemstone is very rare. Faceted pieces, if any exist, would weigh under 1 to 2 carats.
This very rare mineral occurs in small grains. First discovered in Gornaya Shoriya, Russia, no cut specimens exist.
Also known as piemontite, this species is sometimes confused with thulite, a pink manganiferous variety of zoisite. While thulite sometimes appears bright pink, piedmontite typically has a dark brown or reddish color. Thulite can occur in large pieces, but piedmontite seldom occurs in large masses. However, both materials can make lovely cabochons.
Piedmontite: Adams County, Pennsylvania (~4 inches across). Photo ? Joel E. Arem, PhD, FGA. Used with permission.
A very rare, deep emerald-green colored variety of epidote from Myanmar, this material gets its color from chromium.
A popular material for cabochons, unakite, an altered granite, contains green epidote, white to gray quartz, and pink feldspar (orthoclase). The United States produces and exports this widely used lapidary rock, but Ireland, Zimbabwe, and other countries also produce similar material.
“Unakite,” tumble polished stone, by Ra’ike. Licensed under CC By 3.0.
The light, striped lines of “Zebra Stone” from Arizona may contain epidote.
“Rosalinda,” an ornamental rock from Peru, consists of calcite, scapolite, and red epidote-piedmontite.
Another ornamental rock used for carving, “Bowesite,” from Australia, consists of epidote, diopside, grossular garnet, actinolite, and other gem materials.
“Lapis Nevada” consists of pink thulite, yellow-green epidote, green diopside, and white to lavender scapolite.
| ? | Clinozoisite | Epidote | Piedmontite | Hancockite | Allanite |
| Specific Gravity | 3.21-3.38 | 3.38-3.49 | 3.45-3.52 | 4.03 | 3.4-4.2 |
| Optics | ? | ? | ? | ? | ? |
| a | 1.670-1.715 | 1.715-1.751 | 1.732-1.794 | 1.788 | 1.640-1.791 |
| β | 1.675-1.725 | 1.725-1.784 | 1.750-1.807 | 1.81 | 1.650-1.815 |
| γ | 1.690-1.734 | 1.734-1.797 | 1.762-1.829 | 1.83 | 1.660-1.828 |
| 2V | (+) 14-90° | (-) 90-116° | (+) 2-9° | (-) 50° | (+/-) 40-123° |
| Birefringence | 0.005-0.015 | 0.015-0.049 | 0.025-0.073 | 0.042 | 0.013-0.036 |
Sri Lanka produces a yellow-brown epidote with the following properties:
Minas Gerais, Brazil produces cuttable, yellowish-green trichroic crystals. They’re low in iron and have the following properties:
Epidote may have a gray streak. Please note that streak testing may harm or destroy your specimen. Conduct it on a piece of rough, never a finished gem, only as a last resort.
Scientists have synthesized epidotes for petrological research. Clinozoisite has also been created for mineralogical research. However, there is no known jewelry use for these materials.
Epidotes don’t typically receive any treatments or enhancements.
Untersulzbachthal, Austria is the main source of faceting rough.
Other important gem-quality sources include:
“Epidote,” 5.0 x 1.7 x 0.55 cm, Knappenwand, Untersulzbach valley, Salzburg, Austria. ? Rob Lavinsky, www.iRocks.com. Used with permission.
Gavilanes, Baja California, Mexico produces brownish facetable crystals.
Other notable gem-quality sources include:
Various localities throughout the United States produce this material.
Other sources include:
Franklin, New Jersey, the only notable locality, produces small crystals.
Piedmontite occurs in sericite schists in Piemonte, Italy.?Egypt produces a porphyry colored red by piedmontite. California and Arizona also have many producing localities.
Other sources include:
Blue Ridge, Unaka Range, North Carolina produces this lapidary material. Other sources include Virginia and Georgia. In addition, Zimbabwe yields a similar rock.
Unakite occurs in huge blocks weighing many pounds. Gem cutters often cut it into spheres as well as cabs. Facetable epidote is rare over 5 carat sizes. Cut clinozoisite tends to be even smaller. Allanite is rarely ever cut, except as cabochons. Piedmontite is opaque and massive. It’s also cut only as cabochons.
A rare example of a faceted allanite. “Allanite,” 11.9 x 5.8 mm, 3.47 carats, elongated rectangle cut, Brazil. ? Rob Lavinsky, www.iRocks.com. Used with permission.
Some epidotes may have a hardness of 7, on par with quartz. However, most gem materials in this supergroup have a hardness of 6 or softer. Since these gems also have perfect cleavage and sensitivities to heat and acids, clean them only with a soft brush, mild detergent, and warm water. For jewelry use, have these stones set in protective settings and/or reserve them for occasional wear. For more recommendations, consult our gemstone jewelry care guide.
“Epidote (Rough and Cut Set),” 2.1 x 1.6 x 0.8 cm (Crystal), 4.35 ct (Gem), Tormiq, Gilgit-Baltistan (Northern Areas), Pakistan. ? Rob Lavinsky, www.iRocks.com. Used with permission.
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