Dolomite Identification Guide
Identify dolomite by its rhombohedral crystals, slow acid reaction, hardness, and the key tests that separate it from calcite.
Read the full Dolomite encyclopedia entry →
What Dolomite Looks Like
Dolomite is a calcium-magnesium carbonate (CaMg(CO3)2) mineral, and 'dolomite/dolostone' also names the carbonate rock made of it. Crystals are commonly rhombohedra with characteristic curved (saddle-shaped) faces and a pearly to vitreous luster. Colors are usually white, gray, cream, or pinkish-tan, sometimes flesh-pink. The rock form is often sugary-granular and dull.
Key visual cues:
- Color: white, gray, cream, pinkish, tan.
- Luster: vitreous to pearly; saddle crystals look slightly curved.
- Crystal habit: rhombohedral, frequently with curved faces.
- Transparency: translucent to opaque.
Step-by-Step Field-ID Checklist
- Test hardness. Dolomite is Mohs 3.5–4 — slightly harder than calcite (3); a knife scratches it but with more resistance.
- Do the acid test (the key one). Dolomite reacts only weakly and slowly to cold dilute HCl, and usually only when powdered/scratched; calcite fizzes vigorously as a solid surface.
- Look for curved rhombohedral crystals — saddle-shaped faces strongly suggest dolomite.
- Check cleavage — perfect rhombohedral cleavage in three directions.
- Note color — pinkish/tan hues are common in dolomite.
Diagnostic Tests
- Mohs hardness: 3.5–4 (vs calcite's 3).
- Streak: white.
- Cleavage: perfect rhombohedral (three directions, not at 90°).
- Density: ~2.85 g/cm³ (slightly higher than calcite's ~2.71).
- Acid: weak, slow fizz in cold dilute HCl; reacts readily only when powdered or in warm/stronger acid. This is the decisive calcite separator.
- Magnetism: none.
Common Look-Alikes and How to Tell Them Apart
- Calcite: the critical look-alike — calcite fizzes briskly in cold dilute acid as a solid, while dolomite reacts weakly and mostly when powdered. Calcite is also slightly softer (Mohs 3).
- Magnesite (MgCO3): harder edge (3.5–4.5), reacts to acid only when warm/powdered like dolomite; usually whiter and lacks pink saddle crystals — a chemical test may be needed.
- Ankerite (Fe-rich): closely related; browner and may rust on weathering; field separation is difficult.
- Aragonite: same composition as calcite but different habit (needles/sprays) and fizzes briskly; dolomite reacts slowly.
- Marble vs dolomitic marble: check the acid reaction to tell calcitic from dolomitic marble.
Where It Is Found
Dolomite forms by replacement of limestone by magnesium-rich fluids (dolomitization) and as a primary precipitate in evaporitic settings; it also occurs in hydrothermal veins. It is widespread in carbonate sedimentary sequences worldwide — the Dolomite Mountains of Italy, the U.S. Midwest, Switzerland (the type region of Dolomieu's namesake), and Brazil and Mexico for fine crystals. Dolostone forms thick rock units and is quarried for aggregate, magnesium, and flux.
Frequently asked questions
How can you tell if it's dolomite?
Confirm Mohs 3.5–4 hardness, perfect rhombohedral cleavage, often curved saddle-shaped crystals, and especially a weak, slow acid reaction that mainly appears when the mineral is powdered — unlike calcite, which fizzes briskly as a solid.
What is the difference between dolomite and calcite?
Calcite fizzes vigorously in cold dilute acid and is Mohs 3, while dolomite reacts only weakly and slowly (mostly when powdered) and is slightly harder at 3.5–4. The acid test is the quickest separator.
Does dolomite fizz in vinegar or acid?
Only weakly and slowly. A solid dolomite surface barely reacts to cold dilute acid; you usually need to scratch it to a powder or warm the acid to see fizzing, which distinguishes it from calcite.
What does dolomite look like?
It is typically white, gray, cream, or pinkish-tan with a vitreous-to-pearly luster, often forming rhombohedral crystals with curved, saddle-like faces.
Dolomite identified by the community
Recent Dolomite specimens identified with Rock Identifier.
