How to Identify Limestone: What to Look For and Home Tests

Limestone and its relatives formed from shells or from minerals that came out of water, so they react to dilute acid and often contain fossil fragments. The sources we opened give only tests and comparisons for Limestone, not a full property list; see below.

• Dilute HCl test: limestone (mostly calcite) fizzes immediately and vigorously in dilute hydrochloric acid; dolomite must be powdered before it will fizz. Safety: Dilute hydrochloric acid is corrosive. Wear eye protection and gloves, use only a few drops in a ventilated area, avoid skin and eye contact, rinse tools immediately afterward, and do not let minors handle it unsupervised.

• Dolomite / Dolostone: The acid reaction differs in strength: limestone fizzes immediately and vigorously in dilute HCl, while dolomite must be powdered before it fizzes.
• [Marble](/lookup/rocks-and-minerals/marble): Marble forms when limestone undergoes regional metamorphism and recrystallization. Its mineral composition is similar (mostly calcite), but it is denser, coarser-grained, harder, and more durable.

Massive limestone formations near Leadville, Colorado.

Photograph a fresh, unweathered surface in daylight, then a second shot of any crystal faces or broken edge. Include a coin for scale. If you did a streak test, photograph the streak next to the specimen. The app suggests the likely mineral or rock; confirm with hardness and the tests above before you label it.

Limestone itself is made of calcite grains (Mohs hardness 3), but the overall hardness of the rock depends on how well cemented it is, so there is no single value and the field is left blank.