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Slate
Slate is metamorphic (low-grade, foliated). In the hand, it rates 3 to 4 (aggregate) on the Mohs scale, and it runs gray and black. Fine-grained gray rock that splits into flat ringing sheets along perfect parallel planes. Metamorphosed shale; notable: Wales, Spain, Vermont/Pennsylvania (USA).
The geology.
Slate has no single chemical formula. It has no single crystal system. Catalogs list it as metamorphic (low-grade, foliated).
It sits at 3 to 4 (aggregate) on the Mohs scale. The surface reads dull to slightly sheeny. Light behavior is opaque. Specific gravity runs 2.7 to 2.8. It has no diagnostic streak. It breaks with perfect slaty cleavage, splits into sheets.
Fine-grained gray rock that splits into flat ringing sheets along perfect parallel planes.
It reaches buyers as flat sheets, coasters, carved pieces or roofing/decor.
The origins.
Metamorphosed shale; notable: Wales, Spain, Vermont/Pennsylvania (USA).
On value, the database notes: common and very affordable.
Traditional associations.
In energy work traditions, associated with record-keeping and steady order.
We keep this section because people ask. It sits below the geology on purpose, and none of it describes something the stone does.
Spotting the real thing.
Fine-grained gray rock that splits into flat ringing sheets along perfect parallel planes.
| Looks like | How to tell them apart |
|---|---|
| Shale | Shale is softer, crumbles, and smells earthy when wet; slate rings when tapped |
| Schist | Schist sparkles with visible mica; slate is matte |
| Basalt | Basalt is massive without sheet cleavage |
At-home tests.
Before anyone talks you into a lab report, these are the checks worth doing yourself on slate.
| Test | What you should see |
|---|---|
| By eye | Splits into thin flat plates |
| By eye | Faint ring when tapped (vs shale's thud) |
| Scratch check | Knife scratches it |
Anything that scratches, burns or dissolves the piece is a lab job. Get it appraised rather than testing it apart.
Care & handling.
At 3 to 4 (aggregate) on the Mohs scale this is a soft material. Keep it as a display piece, store it apart from harder stones, and skip daily-wear settings.
It breaks with perfect slaty cleavage, splits into sheets, which is worth knowing before you set it or drop it.
Warm water and a soft cloth clean almost everything. Ultrasonic tanks, steamers and household cleaners are where pieces get ruined.
A deeper look.
Extended detail on composition, sourcing, authentication and market value.
The physical record
There is no single formula on record. Crystal system: not applicable. Specific gravity: 2.7 to 2.8. Streak: not applicable. Cleavage and fracture: Perfect slaty cleavage, splits into sheets.
How it reaches the market
Metamorphosed shale; notable: Wales, Spain, Vermont/Pennsylvania (USA).
What it gets confused with
Shale: Shale is softer, crumbles, and smells earthy when wet; slate rings when tapped. Schist: Schist sparkles with visible mica; slate is matte. Basalt: Basalt is massive without sheet cleavage.
What it is worth
Common and very affordable.
One more thing
A good roofing slate outlasts the roofer's grandchildren, Welsh slate roofs run 150+ years.
An education-first guide.
No product links here. We do not sell slate, and a guide that only covers our own shelves would not be much of a guide.
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