igneous
Granite or granitic gneiss
Felsic granitoid, chiefly quartz, feldspar, and mica; possibly weakly foliated gneiss
AI-generated identification · may be incorrectCoarse-grained, speckled white-gray, pink, and dark minerals with a granular texture; likely quartz (Mohs 7), feldspar (Mohs 6), and biotite/hornblende (Mohs 2.5–6). Vitreous quartz and pearly mica may be visible, with no obvious cleavage in the whole specimen; specific gravity about 2.6–2.8.
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Physical properties
Coarse-grained, speckled white-gray, pink, and dark minerals with a granular texture; likely quartz (Mohs 7), feldspar (Mohs 6), and biotite/hornblende (Mohs 2.5–6). Vitreous quartz and pearly mica may be visible, with no obvious cleavage in the whole specimen; specific gravity about 2.6–2.8.
Formation & geological history
Granite crystallizes slowly from silica-rich magma deep underground, typically during Precambrian to Paleozoic crustal events; gneiss is metamorphosed granite. Near 40.1°N, 85.4°W (east-central Indiana), such material is most likely transported glacial erratic or construction/landscaping stone rather than locally exposed bedrock.
Uses & applications
Used extensively as dimension stone, countertops, monuments, aggregate, and decorative landscaping rock. Small pieces have modest educational or collecting value.
Geological facts
The contrasting pale feldspar/quartz and dark mica are characteristic of felsic continental crust. A streak test should be white; dilute acid should produce no bubbling unless carbonate-bearing material is present.
Field identification & locations
Look for interlocking grains and hard, glassy quartz; granite has no consistent layering, whereas gneiss shows aligned dark and light bands. Common regional sources include glacial deposits and imported crushed stone. Estimated value: $1–$10 for an ordinary hand specimen, or up to about $20 if attractive and clearly labeled.
