metamorphic
Quartzite Cobble
Quartzite (SiO2)
Hardness: 7 (Mohs scale), Color: tan, grey, and reddish-brown due to iron staining, Luster: Vitreous to dull, Crystal Structure: Hexagonal (microcrystalline), Cleavage: Indistinct, Specific Gravity: approx 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: tan, grey, and reddish-brown due to iron staining, Luster: Vitreous to dull, Crystal Structure: Hexagonal (microcrystalline), Cleavage: Indistinct, Specific Gravity: approx 2.65
Formation & geological history
Formed through the regional metamorphism of quartz sandstone. Intense heat and pressure cause the quartz grains to recrystallize and fuse together. The rounded shape indicates it was later eroded and transported by water, likely in a river or glacial setting.
Uses & applications
Used primarily as aggregate in construction, road ballast, and occasionally for lapidary work or landscape decoration.
Geological facts
Quartzite is so tough that it often survives the weathering process that destroys the surrounding rock, leading it to be found as isolated rounded cobbles in stream beds far from its source.
Field identification & locations
Identify in the field by its hardness (will easily scratch glass) and its sugary texture on any fresh breaks. Commonly found in riverbeds and glacial tills. Look for the way it breaks through the quartz grains rather than around them.
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