“Millerite,” flexible and soft to the touch, 2.8 x 3.5 x 1.5 cm, Halls Gap, Lincoln Co., Kentucky, USA. © Rob Lavinsky, www.iRocks.com. Used with permission.
Millerite has a rich, attractive yellow color. Massive millerites can sometimes be cut into cabochons but are too soft for jewelry use. However, millerite crystals can also have a striking, hair-like appearance.
| Data | Value |
|---|---|
| Name | Millerite |
| Formula | NiS. |
| Etymology | After mineralogist W. H. Miller, who first studied the crystals. |
| Occurrence | A low-temperature mineral in limestones and dolomites, serpentines, and ore deposits in carbonate rocks. |
| Colors | Brassy and bronze yellow; tarnishes greenish gray. |
| Fracture | Uneven |
| Hardness | 3-3.5 |
| Cleavage | Perfect 2 directions |
| Crystallography | Hexagonal (trigonal). Crystals capillary or acicular; tufted; fibrous; also massive and cleavable. |
| Birefringence | None |
| Pleochroism | Weak. Pale yellow-brown/bright yellow. |
| Optics | Anisotropic, strong. |
| Luster | Metallic. |
| Specific Gravity | 5.3-5.6 |
| Transparency | Opaque. |
“Millerite,” a massive brassy, highly lustrous specimen with intergrown, acicular needles, 3.9 x 3.5 x 2.2 cm, Perseverance Mine (Boulder Perseverance Mine), Kalgoorlie, Kalgoorlie-Boulder City, Goldfields-Esperance region, Western Australia, Australia. © Rob Lavinsky, www.iRocks.com. Used with permission.
Although massive millerite occurs abundantly at certain locations, it generates little interest from collectors. Its very low hardness (3 to 3.5) and perfect cleavage in two directions make it unsuitable for wear. Thus, as gem material for cabochons and decorative objects, it rarely appears on the market.
However, as sprays of very thin capillary or filiform crystals, millerites take on an unusual, almost organic appearance. Sometimes called “hair pyrites,” these millerites indeed resemble tufts of hair growing within geodes. These millerite formations are also flexible and soft.
“Millerite in Geode (Halls Gap, Kentucky, USA) 1” by James St. John. Licensed under CC By 2.0.
Millerites show very weak pleochroism when viewed normally in air. However, in oil, they display more strongly. They show pale yellow-brown color on the ordinary ray and bright yellow color on the extraordinary ray.
Millerite leaves a greenish black streak. Keep in mind that streak testing can destroy the test sample. Conduct this examination only as a last resort for gem identification on rough, never on a finished gem.
Millerite serves as an important ore for the metal nickel. Thus, scientists use synthetic millerite to study the extraction of nickel from this mineral in laboratory settings. However, jewelry use of this synthetic material is unknown.
None known.
Antwerp, New York produces fine sprays of acicular crystals. The Gap Mine in Lancaster County, Pennsylvania produces acicular tufts.
Halls Gap, Kentucky and Missouri yield millerites as tufts of fibers in geodes.
Timagami, Ontario, Canada produces large cleavable masses.
Other notable sources include:
“Millerite,” 2.5 x 2.1 x 1.4 cm, Friedrich Mine, Wissen, Siegerland, Rhineland-Palatinate, Germany. © Rob Lavinsky, www.iRocks.com. Used with permission.
From massive material, gem cutters could cut cabochons or slabs of any size.
You’re more likely to find millerites in gem collections than jewelry collections. See our gemstone jewelry cleaning guide for care recommendations.
“Millerite-220950,” field of view 5mm, Hendrik-Emma dumps, Brunssum, Limburg, Netherlands, by Leon Hupperichs. Licensed under CC By-SA 3.0.
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