mineral and organic gemstone
Turquoise and Spiny Oyster (Heishi style beads)
Turquoise (CuAl6(PO4)4(OH)8·4H2O) and Spondylus (Spiny Oyster shell)
Turquoise: Hardness 5-6 (Mohs), sky blue to greenish-blue, waxy to sub-vitreous luster, triclinic crystal system. Spiny Oyster: Calcite/Aragonite composition, hardness 3.5-4, vibrant red/orange/purple hues, pearly to dull luster.
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Physical properties
Turquoise: Hardness 5-6 (Mohs), sky blue to greenish-blue, waxy to sub-vitreous luster, triclinic crystal system. Spiny Oyster: Calcite/Aragonite composition, hardness 3.5-4, vibrant red/orange/purple hues, pearly to dull luster.
Formation & geological history
Turquoise forms as a secondary mineral in the oxidation zone of copper deposits through hydrothermal precipitation. Spiny Oyster is an organic gemstone formed by marine bivalve mollusks primarily in the Gulf of California.
Uses & applications
Primarily used in jewelry, particularly in Native American (Navajo, Zuni, Santo Domingo) lapidary arts such as Heishi beads and inlay work. Highly valued by collectors of Southwestern art.
Geological facts
This combination of turquoise and shell is traditional to the Pueblo peoples of the American Southwest; the blue represents the sky and water, while the red/orange represents the earth or fire. The turquoise shown appears to be 'stabilized' or 'matrixed' commonly used in bead making.
Field identification & locations
Identify turquoise by its characteristic Robin's egg blue and dark matrix (limonite or sandstone). Identify Spiny Oyster by its striations and vibrant natural pigments. Common in the US Southwest, Mexico, and Iran. Collectors should look for smoothness of the drill holes and color consistency.
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