mineral

Milky and Smoky Quartz Cobble

Silicon Dioxide (SiO2)

Hardness: 7 on Mohs scale; Color: mix of white (milky) and grey/black (smoky); Luster: Vitreous to greasy; Crystal Structure: Hexagonal; Cleavage: None (conchoidal fracture); SG: 2.65

Hardness: 7 on Mohs scale
Color: mix of white (milky) and grey/black (smoky)
Luster: Vitreous to greasy

Identified May 3, 2026More mineral →

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Physical properties

Hardness: 7 on Mohs scale; Color: mix of white (milky) and grey/black (smoky); Luster: Vitreous to greasy; Crystal Structure: Hexagonal; Cleavage: None (conchoidal fracture); SG: 2.65

Formation & geological history

Formed from silica-rich hydrothermal fluids or within igneous pegmatites. This specimen shows significant rounding, suggesting it was transported and eroded in a high-energy water environment like a riverbed or shoreline.

Uses & applications

Quartz is used in glassmaking, electronics (piezoelectric properties), and as an abrasive. While this cobble is not gem-quality, it has decorative and educational value for collectors.

Geological facts

The smoky coloration is caused by natural radiation from surrounding rocks acting on trace aluminum impurities within the crystal structure. It is one of the most abundant minerals on Earth.

Field identification & locations

Identify by its hardness (it will scratch glass), lack of cleavage, and translucent zones. Commonly found in gravel pits, riverbanks, and glacial till. Look for the characteristic glassy luster on broken faces.