igneous
Granitic Pebble (Glacial Erratic)
Granite
Hardness: 6-7 (Mohs scale); Color: Mottled white, grey, and black with brown iron staining; Luster: Vitreous to dull; Crystal structure: Phaneritic (coarse-grained crystals of quartz, feldspar, and mica); Cleavage: Indistinct; Specific Gravity: 2.63–2.75
- Hardness
- 6-7 (Mohs scale)
- Color
- Mottled white, grey, and black with brown iron staining
- Luster
- Vitreous to dull
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Physical properties
Hardness: 6-7 (Mohs scale); Color: Mottled white, grey, and black with brown iron staining; Luster: Vitreous to dull; Crystal structure: Phaneritic (coarse-grained crystals of quartz, feldspar, and mica); Cleavage: Indistinct; Specific Gravity: 2.63–2.75
Formation & geological history
Formed through the slow cooling of silica-rich magma deep underground during the Precambrian or Paleozoic. In this region (Eastern Iowa/Midwest), these stones are glacial erratics transported from the Canadian Shield by Pleistocene ice sheets.
Uses & applications
Used in aggregate for construction, decorative landscaping, and as a durable material for countertops or monuments in larger formats.
Geological facts
This specimen is likely a 'glacial erratic,' a piece of rock that differs from the size and type of rock native to the area in which it rests, carried there by glaciers thousands of years ago.
Field identification & locations
Found in gravel pits, riverbeds, and fields throughout the Driftless Area and surrounding glaciated plains. Identified by its salt-and-pepper appearance and inability to be scratched by a steel pocketknife.
