Brilliant shield-cut bytownite, 21.34 cts, 26.7 x 18.1 mm, Mexico. © The Gem Trader. Used with permission.
Although bytownite occurs throughout the world, this feldspar variety is rarely encountered in gem-quality condition.
Brilliant shield-cut bytownite, 21.34 cts, 26.7 x 18.1 mm, Mexico. © The Gem Trader. Used with permission.
Although bytownite occurs throughout the world, this feldspar variety is rarely encountered in gem-quality condition.
The International Gem Society (IGS) has a list of businesses offering gemstone appraisal services.
| Data | Value |
|---|---|
| Name | Bytownite |
| Is a Variety of | Feldspar |
| Crystallography | Triclinic |
| Refractive Index | 1.561-1.570 |
| Colors | Colorless, white, gray, yellowish, pale yellow, greenish yellow, greenish, reddish brown. |
| Luster | Vitreous, dull. |
| Hardness | 6-6.5 |
| Fracture | Conchoidal to uneven |
| Specific Gravity | 2.72-2.75 |
| Birefringence | 0.009 |
| Cleavage | Perfect 1 direction, Good 2 directions. |
| Dispersion | 0.012 |
| Transparency | Translucent to transparent |
| Optics | α = 1.561; β = 1.565; γ = 1.570. Biaxial (+/-); 2V=86°. |
| Optic Sign | Biaxial +, Biaxial - |
| Luminescence Present | No |
| Luminescence | None |
| Phenomena | May show labradorescence. |
| Formula | (Ca,Na)(Si,Al)4O8 |
| Pleochroism | None |
| Etymology | After Bytown (now known as Ottawa), Canada. |
| Occurrence | In basic plutonic rocks, some metamorphic rocks, and meteorites. |
Bytownite, 2.5 x 1.9 cm, Dorado Mine, Casas Grandes Municipality, Chihuahua, Mexico. Photo by John Sobolewski. Licensed under CC By 3.0.
Bytownite is part of the plagioclase feldspar solid solution series. It has a composition closer to calcium-rich anorthite than sodium-rich albite, about 70-90% to 10-30%, respectively. Bytownites occur in a great variety of rock types and environments, However, transparent crystals are rare.
Some plagioclases, like labradorite, make popular gemstones. Nevertheless, with a hardness of 6 to 6.5 and perfect cleavage in one direction and good or imperfect in two others, bytownites are still susceptible to damage from scratches and blows against hard surfaces. As ring stones, bytownites would need protective settings. Use for earrings, pendants, and brooches would be more advisable.
This 18K gold brooch/pendant features a 3.31-ct bytownite as the butterfly’s body. Sapphire, boulder opals, Gilson opals, kornerupines, and diamonds comprise other parts. Goldsmith, André Enskat; engraver, H. Postler; gem cutter, H. Lorenz. Photo courtesy of liveauctioneers.com and Henry’s Auktionshaus AG.
There are no known synthetic bytownites or treatments for natural specimens.
Chihuahua, Mexico produces gem-quality bytownites.
In the United States, reddish bytownite pebbles from Arizona and New Mexico have been faceted into small gems. Bytownites have also been reported from Plush, Oregon, a well-known locality for facetable labradorite crystals. Some of these feldspars may have a borderline composition and cross over into the bytownite range.
Other notable gem-quality sources include the following:
Due to their cleavage, avoid cleaning bytownites with mechanical cleaning systems, like steam and ultrasound. Clean them only with a soft brush, mild detergent, and warm water. Consult our gemstone jewelry cleaning guide for more recommendations.
Rough and cut set, bytownites, 3.2 x 2.3 x 1.6 cm (crystal),14.23 ct (cushion step-cut gem), Chihuahua, Mexico. © Rob Lavinsky, www.iRocks.com. Used with permission.
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