THOMSONITE: New Mexico (cabochon 23 ~47 mm), Isle Royale, Lake Superior, Michigan (~½ inch each, rough stones). Photo © Joel E. Arem, PhD, FGA. Used with permission.
Thomsonite cabochons take a high polish but are somewhat brittle. These are especially lovely when a pinkish gray eyelike pattern is present, but such material is rare, Lintonite, from Michigan, is translucent and green and is sometimes mistaken for jade. A faceted thomsonite must be considered a great rarity.
| Data | Value |
|---|---|
| Name | Thomsonite |
| Crystallography | Orthorhombic. Crystals prismatic or acicular, and very rare; usually compact, or in radial or fibrous aggregates. |
| Refractive Index | 1.497-1.544 |
| Colors | Colorless, white yellowish, pink, greenish, grayish. A translucent green variety has been called lintonite. |
| Hardness | 5-5.5 |
| Fracture | Uneven |
| Birefringence | 0.021 |
| Cleavage | Perfect 1 direction |
| Stone Sizes | Cabochons up to several inches in length have been cut from material recovered in Michigan at Isle Royale, the best known locality. Large pieces are not abundant, especially with good patterns. Faceted gems are exceedingly rare; gems up to 5 carats from a German locality have been reported to me. |
| Luminescence | Patches of brown and white in LW. |
| Luminescence Present | Yes |
| Luminescence Type | Fluorescent, UV-Long |
| Absorption Spectrum | None. |
| Formula | NaCa2Al5Si5O20 · 6H2O. |
| Optics | a = 1.497-1.530; β = 1.513-1.533; γ = 1.518-1.544. Biaxial(+), 2V= 42-75°. Shadow edge at 1.52-1.54. |
| Optic Sign | Biaxial + |
| Luster | Vitreous to pearly. |
| Specific Gravity | 2.25-2.40 |
| Transparency | Transparent to opaque. |
Optics: a= 1.497—1.530: β=1.513—1.533; γ=1.518-1.544.
Biaxial(+), 2V= 42—75°.
Shadow edge at 1.52-1.54.
Occurrence: Thomsonite is a secondary mineral in lavas and basic igneous rocks.
Oregon; California; Colorado; New Jersey.
Nova Scotia, Canada; Greenland; Ireland; Scotland; Italy; India: Czechoslovakia; Germany.
Isle Royale, Michigan: patterned pebbles.
Stockly Bay, Michigan: lintonite; also at Grand Marais, Cook County, Minnesota (Thomsonite Beach).
Comments: Thomsonite cabochons take a high polish but are somewhat brittle. These are especially lovely when a pinkish gray eyelike pattern is present, but such material is rare, Lintonite, from Michigan, is translucent and green and is sometimes mistaken for jade. A faceted thomsonite must be considered a great rarity.
Thomsonite is pyroelectric.
Name: Thomsonite for Thomas Thomson, the Scottish chemist who first analyzed the material. Lintonite is after a Miss Linton.
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