Oval-cut, pale yellow euclase, 4.26 cts, 12.8 x 9.8 x 5.2 mm, Ouro Preto, Brazil. © ARK Rare Gems. Used with permission.
Although hard enough to be worn safely in jewelry, euclase in beautiful colors is rare over a few carats. It’s also a difficult gem to facet.
| Data | Value |
|---|---|
| Name | Euclase |
| Crystallography | Monoclinic, crystals tabular or prismatic, often well developed. |
| Refractive Index | 1.650-1.676 |
| Colors | Colorless, white, pale blue, pale green, violet, dark blue, yellow. May show blue/colorless or light blue/dark blue color zoning. |
| Luster | Vitreous. |
| Hardness | 6.5-7.5. May be variable within a single crystal. |
| Fracture | Conchoidal |
| Specific Gravity | 2.99-3.10. Colorless = 3.08; dark blue (Zimbabwe) = 3.06-3.13. |
| Birefringence | 0.019-0.024 |
| Cleavage | Perfect 1 direction |
| Dispersion | 0.016 |
| Luminescence | Feeble or none. |
| Luminescence Present | Yes |
| Enhancements | Radiation can turn colorless euclase blue or green. |
| Typical Treatments | Irradiation |
| Transparency | Transparent to translucent. |
| Absorption Spectrum | There are two vague bands at 4680 and 4550; if Cr is present, it may display a characteristic spectrum in the red with a doublet at 7050. |
| Formula | BeAISiO4(OH) |
| Pleochroism | Light colored material may display: bluish gray/light blue/colorless; yellow-green/blue-green/colorless; white-green/yellow-green/blue-green. Zimbabwe dark blue material displays intense pleiochroic colors: azure blue/Prussian blue/greenish-blue. Chromium-colored material may display purple/blue-green/colorless. |
| Optics | a = 1.650-1.652; β = 1.655-1.658; γ = 1.671-1.676. Biaxial (+), 2V = 50°. Dark blue material (Zimbabwe): a = 1.652; β = 1.656; γ = 1.671; Birefringence = 0.019. |
| Optic Sign | Biaxial + |
| Etymology | From the Greek eu for “easy” and klasis for “fracture,” because of its easy cleavage. |
| Occurrence | Euclase is a mineral of granite pegmatites. |
Euclase: Brazil (ca 0.3 to 1.3). Photo © Joel E. Arem, PhD, FGA. Used with permission.
Colorless euclase may not look terribly exciting, but faceted pieces can have exceptional brilliance. Strongly colored euclases in blue or green are prized by collectors of rare gems. Violet is the rarest and most desired color.
With hardness ranging from 6.5 to 7.5, euclases would make good jewelry stones, but they present some challenges for faceters. Their perfect cleavage in one direction makes cutting a bit tricky. Jewelers would also need to set this stone with care, especially in a ring, to avoid placing stress on its cleavage plane.
Gemmy, blue euclases, 2.7 x 2.2 x 1.6 cm, Karoi District, Mashonaland West, Zimbabwe. © Rob Lavinsky, www.iRocks.com. Used with permission.
Euclases may have variable hardness even within the same crystal.
Most euclases receive what color they have from traces of iron. However, some euclases from Colombia receive their greenish blue color from traces of chromium. These stones appear red under a Chelsea filter.
Set of euclases, light grass-green with hints of blue, 1.5 x 0.9 x 0.5 cm, Gachala, Guavio-Guateque Mining District, Boyaca Department, Colombia. © Rob Lavinsky, www.iRocks.com. Used with permission.
No known synthetics.
Be aware that some aquamarines have been misrepresented as much rarer light blue or green euclases to sell for more money. Although euclase can form geologically from the decomposition of beryl and is often found with beryl, they’re distinct mineral species. They have different optical and physical properties.
Radiation treatments can turn colorless euclases green or blue.
Brazil produces most cuttable, gem-quality euclases. In Minas Gerais, Ouro Preto yields fine gem material in good crystals up to a length of about 6 cm. Santana de Encoberto yields material with high birefringence and included crystals of apatite, hematite, rutile, and zircon. Boa Vista also yields cuttable crystals.
Bi-color colorless/blue euclase, 0.45 cts, 5 x 4 mm, emerald cut, Brazil. © The Gem Trader. Used with permission.
Miami, Zimbabwe produces cuttable crystals in shades of intense, cobaltian blue hue, some look like fine sapphires.
Other notable gem-quality sources include the following:
Oval-cut, pale yellow euclase, 3.60 cts, 11.2 x 8.8 x 5.7 mm, Russia. © ARK Rare Gems. Used with permission.
Crystals most commonly occur in small sizes, about 1” for colorless material. Most euclases are, in fact, colorless. Strongly colored material is very rare. Blue and green gems are scarce over 2-3 carats. Some Brazilian violet crystals reportedly could yield stones up to 10 carats.
Colorless gems over 5-6 carats are also rare, although stones up to about 20 carats have been cut. Gems reported over 50 carats are museum pieces.
Due to their perfect cleavage, don’t clean euclases with mechanical systems. Use only warm water, mild detergent, and a soft brush. Use protective settings for rings and make sure you discuss with your jeweler how best to set any euclases.
For more cleaning recommendations, consult our gemstone jewelry care guide.
Euclase: Brazil (5.49, 1.34). Photo © Joel E. Arem, PhD, FGA. Used with permission.
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