metamorphic
Slate
Metamorphic Pelitic Rock
Hardness: 3-4 on the Mohs scale; Color: typically dark gray to blue-gray, but can be green, red, or purple; Luster: dull to earthly; Crystal structure: very fine-grained, foliated with excellent slaty cleavage; Specific gravity: 2.7-2.8
- Hardness
- 3-4 on the Mohs scale
- Luster
- dull to earthly
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Physical properties
Hardness: 3-4 on the Mohs scale; Color: typically dark gray to blue-gray, but can be green, red, or purple; Luster: dull to earthly; Crystal structure: very fine-grained, foliated with excellent slaty cleavage; Specific gravity: 2.7-2.8
Formation & geological history
Formed by the low-grade regional metamorphism of mudstone, shale, or sometimes volcanic ash, under relatively low temperatures and directed pressures over hundreds of millions of years.
Uses & applications
Historically used for roofing, flooring, blackboards, billiard tables, and electrical panels; currently popular in architectural paving, landscaping, and upscale tableware.
Geological facts
Slate's prominent slaty cleavage allows it to be split into thin, durable sheets, which made it a primary material for roofing and writing tablets for centuries.
Field identification & locations
Can be identified in the field by its fine-grained texture, dull luster, and characteristic ability to split into thin, flat, smooth sheets along cleavage planes. Common in mountainous regions and areas with ancient orogenic activity.
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