Marble

Hardness: 3-4 on Mohs scale. Color: Pure marble is white, but impurities can result in various colors like grey, pink, green, yellow, black or variegated (veined). Luster: Typically sub-vitreous to dull in masses, but can be vitreous on crystal faces. Crystal structure: Granular, interlocking calcite or dolomite crystals. Cleavage: Perfect rhombohedral in individual calcite/dolomite crystals, but not typically observed in the rock mass. Specific gravity: 2.7-2.9.

Marble forms from the metamorphism of sedimentary carbonate rocks, most commonly limestone or dolostone. This process occurs when these rocks are subjected to high temperatures and pressures, such as during regional metamorphism in convergent plate boundaries or contact metamorphism near igneous intrusions. The original carbonate minerals recrystallize into a mosaic of interlocking crystals. The streaks or veins often seen in marble are typically due to mineral impurities like clay, silt, sand, iron oxides, or chert, which are remobilized and recrystallized during metamorphism. Geological age: Marbles can range in age from Precambrian to Cenozoic, depending on the age of the original sedimentary rock and the timing of the metamorphic event.

Industry: Dimension stone for buildings, sculptures, monuments, flooring, countertops, wall cladding, and decorative items. Construction: Crushed marble is used as aggregate in civil engineering projects, such as roadbeds and foundations, and as a filler in paints, plastics, and paper. Jewelry: While not typically used as a gemstone itself, highly polished, desirable varieties are used in decorative objects and sometimes in artisanal jewelry. Art: Highly prized for sculpture due to its relative softness and ability to take a fine polish.

Many of the world's most famous sculptures, such as Michelangelo's David and the Venus de Milo, are carved from marble. The Parthenon in Greece is another iconic structure built from marble. The term 'marble' is often used commercially to refer to any stone that can take a high polish, but geologically it specifically refers to metamorphosed limestone or dolostone. It reacts with acids, effervescing when a drop of dilute hydrochloric acid is applied, due to its calcite content.

Identification in the field: Look for interlocking crystalline texture, often with a sugary appearance. It will scratch easily with a knife (hardness 3-4). A definitive test is the acid test: it will fizz vigorously when a drop of dilute acid (like vinegar or weak HCl) is applied, indicating the presence of calcite or dolomite. Common locations: Italy (Carrara marble), Greece (Pentelic marble), Vermont and Georgia in the USA, Turkey, China, and India. Tips for collectors: Collect pieces that show interesting coloration, banding, or veining. Be aware that polished samples are more common due to its widespread use as a building and decorative material.