sedimentary

Black Dolomite

Bituminous Dolomite (CaMg(CO3)2)

Hardness: 3.5-4.0 (Mohs scale); Color: Dark gray to black; Luster: Sub-vitreous to dull; Crystal structure: Trigonal (often microcrystalline); Cleavage: Perfect rhombohedral (not visible in rounded pebbles); Specific Gravity: 2.8-2.9.

Hardness: 3
Color: Dark gray to black
Luster: Sub-vitreous to dull

Identified July 1, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed by the chemical alteration (dolomitization) of limestone by magnesium-rich groundwater, with the dark color derived from organic matter or bitumen. The specimen is a water-worn river or beach pebble.

Uses & applications

When found as large deposits, it is used for crushed stone in construction, as a source of magnesium, and for soil pH neutralization. As a pebble, it is primarily a collectable or used in landscaping.

Geological facts

Dark dolomite is often mistaken for meteorite due to its dark surface 'crust-like' appearance caused by weathering or organic staining. It will react very weakly with cold dilute hydrochloric acid unless powdered.

Field identification & locations

Identify in the field by its dark color, relative softness (scratched by a copper coin), and lack of magnetic properties. Common in the region of Visakhapatnam (17.7°N, 83.3°E) within sedimentary and metasedimentary sequences.