metamorphic
Quartzite (Water-worn Cobble)
Quartzite (Composition: 90%+ SiO2)
Hardness: 7 on Mohs scale; Color: Tan, beige, or pale pink due to iron oxide impurities; Luster: Vitreous to waxy; Structure: Granoblastic (interlocking quartz grains); Specific Gravity: 2.63-2.65.
- Hardness
- 7 on Mohs scale
- Color
- Tan, beige, or pale pink due to iron oxide impurities
- Luster
- Vitreous to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, beige, or pale pink due to iron oxide impurities; Luster: Vitreous to waxy; Structure: Granoblastic (interlocking quartz grains); Specific Gravity: 2.63-2.65.
Formation & geological history
Formed through the regional metamorphism of quartzose sandstone. The original sand grains are recrystallized under high heat and pressure, fusing them into a dense, hard rock. This specific specimen has been further shaped by fluvial (river) or glacial erosion, resulting in its smooth, rounded 'river rock' texture.
Uses & applications
Used as aggregate in construction, railway ballast, and road base due to its extreme durability. Highly polished specimens are used in landscaping, decorative fountains, and sometimes as 'worry stones' in the lapidary market.
Geological facts
Quartzite is so hard and dense that when it breaks, it fractures across the original sand grains rather than around them, unlike its parent rock, sandstone. It is one of the most resistant rocks to chemical and physical weathering.
Field identification & locations
Identify in the field by its inability to be scratched by a steel knife (hardness check) and its grainy, crystalline appearance beneath a smooth surface. Commonly found in riverbeds, glacial tills, and ancient mountain belts. To test, it will scratch glass easily.
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