4.2 Pegmatitic tourmaline-muscovite granite. Custer, South Dakota. Gray, glassy quartz, black to silvery-white muscovite mica, and black tourmaline decorate a large, orange-pink crystal of alkali (K-rich) feldspar.

Image 7 of 156

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4.2 Pegmatitic tourmaline-muscovite granite. Custer, South Dakota. Gray, glassy quartz, black to silvery-white muscovite mica, and black tourmaline decorate a large, orange-pink crystal of alkali (K-rich) feldspar.

4.2 Pegmatitic tourmaline-muscovite granite. Custer, South Dakota. Gray, glassy quartz, black to silvery-white muscovite mica, and black tourmaline decorate a large, orange-pink crystal of alkali (K-rich) feldspar.

Image 7 of 156

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