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. |
| 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. As a gemstone, however, it 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.
Colemanites are both pyroelectric and piezoelectric.
Since colemanite is rarely faceted, its birefringent facet doubling is rarely seen. This large, eye-clean, custom shield-cut specimen displays the 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.
No 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 have considerable heat sensitivity, so any setting in metal jewelry should be done very carefully. If worn as jewelry, they require protective settings. These gems are also slightly soluble in water. Don’t soak these gems during cleaning and make sure they’re 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 rough and cut set features a cluster of tabular colemanite crystals and a faceted, 2.47-ct, 8 mm colemanite. Boron, Kramer District, Kern Co., California, USA. © Rob Lavinsky, www.iRocks.com. Used with permission.
Turkey’s gemstones include well-known species such as diaspore, chalcedony, and jade, as well as unusual specimens of lesser-known gems. Learn…
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