metamorphic
Grey Slate
Foliated Metamorphic Rock (formed by low-grade regional metamorphism of shale or mudstone)
Hardness: 2.5–4.0 on Mohs scale; Color: typically dark grey to charcoal; Luster: dull to sub-glassy; Crystal structure: very fine-grained (microscopic crystals of quartz, mica, and chlorite); Cleavage: perfect slaty cleavage allowing it to split into thin flat sheets; Specific Gravity: 2.7–2.8
- Hardness
- 2
- Color
- typically dark grey to charcoal
- Luster
- dull to sub-glassy
Identified More metamorphic →
Identify your own rocks.
Get a report just like this from any photo, free.
Physical properties
Hardness: 2.5–4.0 on Mohs scale; Color: typically dark grey to charcoal; Luster: dull to sub-glassy; Crystal structure: very fine-grained (microscopic crystals of quartz, mica, and chlorite); Cleavage: perfect slaty cleavage allowing it to split into thin flat sheets; Specific Gravity: 2.7–2.8
Formation & geological history
Slate is formed through low-grade regional metamorphism, where sedimentary rocks like shale are subjected to high pressure and low temperatures. This process usually occurs during mountain-building events (orogenies) several hundred million years ago, causing clay minerals to recrystallize into parallel plates.
Uses & applications
Used extensively in construction for roofing shingles, floor tiles, and billiard table tops. It is also used as decorative stone in landscaping and for high-quality blackboards or whetstones.
Geological facts
Slate is famous for its 'slaty cleavage,' which is a physical property independent of the original sedimentary bedding. Historically, it was the primary material for school writing tablets before paper became cheap and widely available.
Field identification & locations
Identify in the field by its fine grain and ability to split into thin, smooth layers with a dull ring when struck. Commonly found in folded mountain belts like the Appalachian Mountains in the US, North Wales in the UK, and parts of Brazil and China.
More like this
