Freeform ceruleite cabochon, 24 cts, 30 x 18 x 10.3 mm, Chile. © 49erMinerals. Used with permission.
A little-known gem material of truly exquisite color, sky-blue ceruleite takes a very high polish easily and quickly. However, fine, solid, cuttable pieces are extremely rare.
| Data | Value |
|---|---|
| Name | Ceruleite |
| Crystallography | Triclinic. Usually massive, compact, earthy. |
| Refractive Index | 1.60 |
| Colors | Turquoise-blue shades, cerulean blue. |
| Luster | Earthy, dull. |
| Hardness | 5 - 6 |
| Specific Gravity | 2.7-2.8 |
| Enhancements | Plastic impregnation. |
| Typical Treatments | Infusion/Impregnation |
| Transparency | Translucent to opaque. |
| Formula | Cu2Al7(AsO4)4(OH)13 |
| Optics | Mean index ~ 1.60. Very fine grained. |
| Etymology | From the Latin caerulea for “sky blue,” in allusion to its color. |
| Occurrence | Sedimentary material formed in the vicinity of copper deposits, like turquoise. |
| Birefringence | None |
Ceruleite: Arizona (specimens each, ~ 1 inch across). Photo © Joel E. Arem, PhD, FGA. Used with permission.
Polished ceruleite gems can have a far deeper blue color than that of the rough nodules. Due to its rarity, good, cuttable material is quite expensive. You’ll see few cut stones, even in museum collections.
This gemstone leaves a bluish white streak. However, keep in mind that streak testing ranks as a destructive test. Conduct it on a piece of rough, never a finished gem, only as a last resort.
The pastel blue ceruleites on this limonite vug have formed as needle-like acicular crystals. Peacock Mine (South Peacock; Darland; Peacock & South Peacock), Cuprum, Seven Devils District, Adams Co., Idaho, USA. © Rob Lavinsky, www.iRocks.com. Used with permission.
Ceruleite’s porosity poses a major problem for jewelry use, rendering the material too soft and fragile for cutting and wear. Plastic impregnation can solve this problem. Such impregnated material has a specific gravity of 2.58.
The type locality for this gem is the Emma Luisa Mine in Guanaco, Chile. Chile remains an important source for this gem.
Southern Bolivia has produced cabbing material of fine color. However, the total amount of this yield may not exceed several hundred pounds.
Other notable gem sources include:
A rare mineral, schlossmacherite occurs only in Chile and sometimes in association with ceruleite. This oval-cut cabochon looks like a robin’s egg, combining schlossmacherite’s pistachio green with ceruleite’s bright blue. 9.57 cts, 16.4 x 11.9 mm, Guanaco, Chile. © The Gem Trader. Used with permission.
Nodules usually range in size from less than 1 inch up to several inches. Typically small, this material yields cabochons only.
You’re more likely to find ceruleites, if at all, in mineral collections than in jewelry collections. These gemstones have a lower hardness (5-6) than more common jewelry stones such as quartz. Therefore, store them separately from other gems to avoid contact scratches and use protective settings for ring wear. However, pendant and earring use should pose fewer risks.
Avoid cleaning ceruleites with any mechanical systems. Clean these gems only with a soft brush, mild detergent, and warm water. Don’t use any other cleaning solutions, since ceruleites may react to chemicals in them. Wipe pieces with a damp cloth after wearing them. Consult our gemstone jewelry cleaning guide for more recommendations.
25 cabochon-cut ceruleites, 102 cts total, Chile. (For comparison, the penny measures 19 mm across). © 49erMinerals. Used with permission.
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