Rock & Mineral Encyclopedia

Search and identify 1,000+ rocks, minerals, crystals, and gemstones — with properties, formation, colors, hardness, and how to tell them apart.

Shale

The most common sedimentary rock, a fissile mudrock of compacted clay and silt that splits into thin layers.

A dark, organic-rich shale loaded with kerogen and bitumen that can yield oil and gas, often finely laminated and combustible.

A fine-grained sedimentary rock rich in solid organic matter (kerogen) that yields oil and gas when heated.

Dark, organic-rich fine-grained sedimentary rock formed in oxygen-poor waters, often a source rock for oil and gas.

Quartz colored or coated by golden iron oxides such as limonite or goethite, giving a warm sunlit yellow glow.

Natural emerald crystals still embedded in their host rock, prized as mineral specimens that show how the gem grew in place.

A fine-grained, sodium-rich contact-metasomatic rock formed where shale is albitized next to intruding diabase or spilite.

A fine-grained sedimentary rock of compacted clay and silt that, unlike shale, breaks in blocks rather than thin layers.

A combustible black sedimentary rock formed from ancient plant matter and burned for centuries as a primary fossil fuel.

The lowest rank of coal, a soft brown carbon-rich rock formed from compacted peat, used mainly for electricity generation.

A lightweight black organic gemstone formed from fossilized wood under pressure, a type of lignite long used in mourning jewelry.

A foliated metamorphic rock of interlayered quartz and mica, producing a sparkling, easily split rock from metamorphosed sandy shales.

Hardened, fine-grained mudrock intermediate between shale and slate, dense and non-fissile, often carved into ornaments.

A rare boron-bearing feldspar, the boron analogue of albite, first found in oil-shale nodules of the Green River Formation.