Prehnite Value, Price, and Jewelry Information

Prehnite: Mexico (2.47), Australia (7.65). Photo © Joel E. Arem, PhD, FGA. Used with permission.

Prehnite

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 Information

Data Value
Name Prehnite
Colors Pale green, dark green, yellow, yellowish green, gray, white, colorless.
Crystallography Orthorhombic. Crystals prismatic and tapering, rare; massive, in druses and crusts, stalactitic.
Refractive Index 1.611-1.665
Luster Vitreous to pearly.
Hardness 6-6.5
Fracture Uneven
Specific Gravity 2.80-3.00; gem material usually 2.88-2.94.
Birefringence 0.021-0.033
Cleavage Distinct 1 direction
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.
Luminescence Present Yes
Luminescence Type Fluorescent, UV-Long, X-ray Colors
Transparency Transparent to translucent.
Absorption Spectrum Not diagnostic.
Phenomena Chatoyancy.
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.
Optic Sign Biaxial +

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; Russia; Czech Republic; Slovakia; 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.

by Joel E. Arem, Ph.D., FGA