Prehnite is popular as a cabochon material among hobbyists because of its lovely green and blue-green to yellow colors. Completely transparent material is extremely rare but might be found in crystals from Asbestos, Quebec. Yellowish to greenish translucent material from Australia has been faceted and makes a striking cut gemstone with a rich color and interesting appearance, with a soft, velvety look. Some catseye stones have also been reported. Material from Scotland has yielded cuttable fragments, but such faceted gems are rather small (under 5 carats).
Prehnite Value
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Prehnite Information
| Data | Value |
| Name | Prehnite |
| Crystallography | Orthorhombic. Crystals prismatic and tapering, rare; massive, in druses and crusts, stalactitic. |
| Colors | Pale green. dark green, yellow, yellowish green, gray, white, colorless. |
| Luster | Vitreous to pearly. |
| Hardness | 6-6.5. |
| Density | 2.80-3.00; gem material usually 2.88-2.94. |
| Birefringence | 0.02l—0.033. |
| Cleavage | Distinct l direction. Fracture uneven. Brittle. |
| Stone Sizes | Large masses, up to several tons in size have been encountered in New Jersey traprocks. Single masses weighing 400 pounds have been collected. Australia and other localities produce translucent material that yields interesting faceted gems up to about 30 carats. |
| Luminescence | May be dull brownish yellow in UV and X-rays. |
| Spectral | Not diagnostic. |
| Formula | Ca2Al2Si3O10(OH)2 + Fe. |
| Pleochroism | None. |
Optics: a= 1.611—1.632; β=1.615-1.642; γ=1.632-1.665.
Biaxial (+), 2V = 65-69°. Usually refractometer gives shadow edge about 1.63.
Note: Faceted material from Australia, indices 1.618/1.625/1.648; birefringence 0.030. Values for optical constants increase with increasing iron content.
Occurrence: A low-temperature mineral occurring by deposition from ground waters in basaltic rocks associated with zeolites; hydrothermal crystals, in cavities in acid
igneous rocks; in serpentine rocks due to late-stage mineralization.
California; Colorado; Michigan; Massachusetts;
Connecticut.
France; Italy; Austria; Germany; USSR; Czechoslovakia; South Africa; Pakistan; New Zealand.
New Jersey: in basalts, associated with zeolites.
Fairfax Quarry, Centreville, Virginia: fine green material.
Asbestos, Quebec, Canada: in acidic dikes, in crystals up to 3 inches long (colorless).
Scotland: facetable.
Australia: facetable.
Comments: Prehnite is popular as a cabochon material among hobbyists because of its lovely green and blue-green to yellow colors. Completely transparent material is extremely rare but might be found in crystals from Asbestos, Quebec. Yellowish to greenish translucent material from Australia has been faceted and makes a striking cut gemstone with a rich color and interesting appearance, with a soft, velvety look. Some catseye stones have also been reported. Material from Scotland has yielded cuttable fragments, but such faceted gems are rather small (under 5 carats).
Name: After Colonel Hendrik von Prehn, who first found the material on the Cape of Good Hope.
