Light brown, round brilliant herderite, 1.57 cts, 7.3 mm, Brazil. © The Gem Trader. Used with permission.
Herderite is a rare collector’s gem, especially in larger sizes. Although too soft for daily jewelry wear, this material is attractive when cut and can show a wide range of colors.
| Data | Value |
|---|---|
| Name | Herderite |
| Crystallography | Monoclinic. Crystals stout or prismatic; tabular; crusts. |
| Refractive Index | 1.591-1.624 |
| Colors | Colorless, pale yellow, greenish white, pink, green, violet. |
| Luster | Vitreous |
| Hardness | 5-5.5 |
| Fracture | Conchoidal |
| Specific Gravity | 2.95-3.02 |
| Birefringence | 0.028-0.030 |
| Cleavage | Indistinct, Interrupted on {110} |
| Dispersion | 0.017 |
| Luminescence | Pale green in LW and SW; also stronger violet in LW. Orange fluorescence in X-rays with persistent phosphorescence. |
| Luminescence Present | Yes |
| Luminescence Type | Fluorescent, Phosphorescent, UV-Long, UV-Short, X-ray Colors |
| Transparency | Translucent to transparent |
| Absorption Spectrum | Not diagnostic. |
| Formula | CaBePO4(F,OH). |
| Pleochroism | None or weak |
| Optics | a = 1.591-1.594; β = 1.611-1.613; γ = 1.619-1.624. Biaxial (- or +), 2V = 67-75°. See “Identifying Characteristics” below. |
| Optic Sign | Biaxial +, Biaxial - |
| Etymology | After S. A. W. von Herder, a mining official in Freiburg, Germany (1776–1838). |
| Occurrence | Late stage hydrothermal mineral in granite pegmatites. |
Herderites: Brazil (9.6, 3.65, 9.25). Photo © Joel E. Arem, PhD, FGA. Used with permission.
Herderite belongs to the herderite-hydroxylherderite mineral series as the fluorine (F)-dominant end member. If hydroxyl (OH) dominates, the specimens are then called hydroxylherderites. Herderites are rarer than hydroxylherderites, and some gems may be misidentified as their hydroxyl-dominant brethren.
Greenish yellow, round brilliant hydroxylherderite, 1.82 cts., 8 mm, Near Ouro Verde, Minas Gerais, Brazil. © The Gem Trader. Used with permission.
With a hardness of 5 to 5.5, herderites would need protective settings for use as ring stones. They’ll scratch easily, so it’s best to reserve them for occasional wear. Other jewelry uses, such as earrings and brooches, would probably pose fewer risks.
Of course, herderites are rare minerals and aren’t well-known by gem enthusiasts, so they rarely appear for sale as faceted pieces. Clean and large stones suitable for cutting are also very hard to find. However, more gem material may come on the market from Brazilian sources.
Green herderite, 6.92-ct, 12.7 x 10.5 x 6.0 mm, Brazil. © ARK Rare Gems. Used with permission.
Some herderites can have a biaxial (+) optic sign. In general, however, OH-dominant specimens will have a positive sign and F-dominant specimens a negative sign.
The refractive indices (RI) of these gems systematically decrease as their F content increases. Thus, RIs in the lower range may indicate herderites.
The green gem from Brazil in Dr. Arem’s photo has the following properties:
No known synthetics or enhancements.
Brazil and Pakistan yield gem-quality herderites as well as hydroxylherderites. Minas Gerais, Brazil has produced crystals up to nearly fist size. Their colors include colorless and pink as well as some green and violetish.
The following locations may produce hydroxylherderites:
Green herderites, Shigar Valley, northeast of Skardu, Baltistan, Pakistan. © Rob Lavinsky, www.iRocks.com. Used with permission.
Faceted gems from Maine usually weigh from 1 to 5 carats and show pale colors or appear colorless. Herderites from Brazil, however, have stronger colors. Faceters may cut stones up to 25 or 30 carats from larger crystals.
Due to their relatively low hardness, don’t clean herderites through mechanical processes, such as steam or ultrasonic systems. Instead, use a soft brush, mild detergent, and warm water. Avoid storing them with other harder, more common gemstones to avoid contact scratches. Consult our gemstone jewelry cleaning guide for more recommendations.
Yellowish green, rectangular step-cut hydroxylherderite, 5.66 cts, 11.5 x 10 mm, Minas Gerais, Brazil. © The Gem Trader. Used with permission.
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