Colorless, round brilliant colemanite, 0.39 cts, 5 mm, Billie Mine, Boron, California. © The Gem Trader. Used with permission.
Colemanite is an abundant mineral, and transparent material isn’t rare. However, gem cutters rarely facet these typically pale stones. Difficult to cut and wear, colemanites are better suited for collectors of unusual gemstones.
| Data | Value |
|---|---|
| Name | Colemanite |
| Crystallography | Monoclinic. Crystals are equant, prismatic, pseudorhombohedral; massive, cleavable; granular, and as aggregates. |
| Crystallographic Forms | |
| Refractive Index | 1.586-1.614 |
| Colors | Colorless, white, grayish, yellowish white, light amber-orange, brown. |
| Luster | Vitreous to adamantine |
| Hardness | 4.5 |
| Fracture | Subconchoidal to uneven |
| Specific Gravity | 2.42 |
| Birefringence | 0.028 |
| Cleavage | Perfect 1 direction |
| Dispersion | Weak |
| Heat Sensitivity | Very heat sensitive |
| Luminescence | May fluoresce and phosphoresce strong yellowish white or greenish white. |
| Luminescence Present | Yes |
| Luminescence Type | Fluorescent, Phosphorescent |
| Transparency | Translucent to transparent |
| Absorption Spectrum | Not diagnostic |
| Formula | Ca2B6O11 · 5H2O |
| Pleochroism | None |
| Optics | a = 1.586; β = 1.592; γ = 1.614. Biaxial (+), 2V ~ 55°. |
| Optic Sign | Biaxial + |
| Etymology | After William T. Coleman, owner of the mine where the mineral was first found. |
| Occurrence | In saline lake deposits in arid regions. |
Colemanite: Boron, California (26.50). Photo © Joel E. Arem, PhD, FGA. Used with permission.
Colemanite serves as an ore for boron and has many industrial uses.
Colemanite has weak dispersion, so it shows little fire, and normally occurs colorless. Since it also has perfect cleavage and a hardness of only 4.5, this fragile gem would make an unlikely choice for a jewelry stone. Ring use would require a protective setting. Other uses, such as in earrings, pendants, or brooches, would be more practical.
Colemanites have considerable heat sensitivity, so jewelers should take great care placing these gems in metal settings.
Colemanite: California (1.4). Photo © Joel E. Arem, PhD, FGA. Used with permission.
Colemanites are both pyroelectric and piezoelectric. They generate an electric current when heated or placed under pressure, respectively.
Faceted colemanites may show facet doubling due to their birefringence.
This large, eye-clean, custom shield-cut colemanite displays the facet doubling effect very nicely. 9.33 cts, 14 x 12.1 mm, Boron, California. © The Gem Trader. Used with permission.
Scientists have synthesized colemanites for a variety of research projects, including studies of their electrical properties. However, there’s no known jewelry use for this lab-created material.
Colemanites don’t usually receive any known gemstone treatments.
The southwestern United States, particularly Boron and Death Valley, California, produces beautiful colemanite crystals. Other notable sources include Argentina, Kazakhstan, and Turkey.
This crystal specimen contains gemmy, honey brown colemanites with “second generation” colorless colemanites on their edges. 5.4 x 5.0 x 3.7 cm, Kestelek Mine, Mustafa Kemalpafla, Bursa Province, Marmara Region, Turkey. © Rob Lavinsky, www.iRocks.com. Used with permission.
Large crystals and masses could yield gems of 50-100 carats. Crystals normally range up to about 1 inch in size.
Colemanites are slightly soluble in water. Don’t soak these gems during cleaning. Make sure to pat them dry before storing. Clean them only with a soft brush, mild detergent, and warm water. For more care recommendations, consult our gemstone jewelry cleaning guide.
This colemanite rough and cut set features a cluster of tabular crystals and a faceted, 2.47-ct, 8 mm gemstone. Boron, Kramer District, Kern Co., California, USA. © Rob Lavinsky, www.iRocks.com. Used with permission.
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