Brilliant oval-cut milarite, 2.07 cts, 9.2 x 7.3 mm, Brazil. © The Gem Trader. Used with permission.
Very rare milarite crystals can occur in green and yellow colors. Transparent material can yield small but pleasant looking faceted gems for collectors.
| Data | Value |
|---|---|
| Name | Milarite |
| Crystallography | Hexagonal. Crystals prismatic and tabular; in grains. |
| Crystallographic Forms | |
| Refractive Index | 1.529-1.551 |
| Colors | Colorless, white, pale green, yellowish, yellowish green. |
| Hardness | 5.5-6 |
| Fracture | Conchoidal to uneven |
| Birefringence | 0.003 |
| Cleavage | None |
| Luminescence | Some specimens may fluoresce and phosphoresce under SW and LW UV. (See “Identifying Characteristics” below). |
| Luminescence Present | Yes |
| Absorption Spectrum | Not diagnostic |
| Formula | K2Ca4Be4Al2Si24O60·H2O |
| Optics | o = 1.532-1.551; e = 1.529-1-548. Uniaxial (-). |
| Optic Sign | Uniaxial - |
| Luminescence Type | Fluorescent, Phosphorescent, UV-Long, UV-Short |
| Luster | Vitreous |
| Specific Gravity | 2.46-2.61 |
| Transparency | Transparent to opaque |
| Pleochroism | None |
| Etymology | After the Val Milar, Switzerland, because the mineral was (mistakenly) thought to have occurred there. |
| Occurrence | In vugs in granites and syenites; hydrothermal veins. |
Cluster of parallel-growth milarites, 1.7 x 1.5 x 1.2 cm, Milarite locality, Rossing Mountains Area, Erongo Region, Namibia. © Rob Lavinsky, www.iRocks.com. Used with permission.
Milarite belongs to the mineral group named after it. Sometimes called the osumilite group, this group also includes sugilite and other rarely faceted minerals, such as poudretteite and sogdianite.
Milarite was originally known as a green mineral, until fine yellow crystals were discovered in Mexico in 1968.
Some milarites may luminesce. Shortwave (SW) ultraviolet (UV) light may cause bluish white or greenish white fluorescence and phosphorescence, while longwave (LW) UV light may cause medium chalky green fluorescence and weak phosphorescence.
Yellow-green milarites on white valencianite, milarite crystals up to 0.6 cm across, Valenciana Mine, Guanajuato, Mun. de Guanajuato, Guanajuato, Mexico. © Rob Lavinsky, www.iRocks.com. Used with permission.
No known gemstone treatments or enhancements.
Guanajuato, Mexico produces yellow and yellow-green gem-quality crystals on matrix, and some larger Mexican crystals have transparent areas suitable for cutting.
Minas Gerais, Brazil and Tsumeb and the Erongo region in Namibia have also yielded small, facetable material.
Milarite: Tsumeb, Namibia (0.53 cts). Photo © Joel E. Arem, PhD, FGA. Used with permission.
Other notable crystal sources include the following locations:
Milarite crystal, Venero 1 Quarry, Cadalso de los Vidrios, Madrid, Spain. Photo by Christian Rewitzer. Licensed under CC By-SA 3.0.
Crystals can occur up to about 4 cm across, but facetable areas in such crystals are very small. Thus, stones over 1 carat could be considered large for the species.
Freeform-cut milarite, 0.98 cts, 8.9 x 6.1 mm, Minas Gerais, Brazil. © The Gem Trader. Used with permission.
With a hardness of 5.5 to 6 and no cleavage, milarites could make suitable jewelry stones if worn with protective settings. However, you’re more likely to find them — if at all — in mineral collections than jewelry collections.
Stay on the safe side and refrain from cleaning these rare gems in mechanical systems. Instead, use a soft brush, mild detergent, and warm water. Consult our gemstone jewelry care guide for more recommendations.
Milarite rough and cut set (crystal: 1.4 x 1.3 x 1.3 cm; emerald-cut gem: 1.38 cts), Jaguaracu, Minas Gerais, Brazil. © Rob Lavinsky, www.iRocks.com. Used with permission.
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