Prehnite Value, Price, and Jewelry Information

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

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
Crystallography	Orthorhombic. Crystals prismatic and tapering, rare; massive, in druses and crusts, stalactitic.
Refractive Index	1.611-1.665
Colors	Pale green, dark green, yellow, yellowish green, gray, white, colorless.
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.

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

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