Aragonite

Aragonite, like calcite, is made of calcium carbonate (CaCO₃), but its atoms are arranged differently, so it is a separate mineral. Its crystals are mostly needle- or column-shaped. It precipitates from sea and fresh water, forms the shells of many molluscs and the skeletons of corals, and builds branching stalactites in caves. It is a little harder than calcite (3.5–4) and heavier.

Aquaria: Aragonite sand and rock are used to recreate coral-reef conditions and to keep the water's pH in balance. Ornaments: Banded aragonite is used as “onyx marble” for vases and ornaments. According to Kunz, the ancient Mexicans made figures, ornaments and beads from aragonite stalagmite. Collecting: Clusters of needle- and branch-shaped crystals, and twinned crystals, are sought by mineral collectors.

It has a hardness of 3.5–4 on the Mohs scale. A fingernail cannot scratch it, but a steel knife can. Its specific gravity is 2.93–2.95 g/cm³.

Its best-known localities: Molina de Aragón, Guadalajara, Spain; Bahama Banks, The Bahamas; Erzberg, Eisenerz, Styria, Austria; Agrigento (Racalmuto, Cianciana), Sicily, Italy; Tsumeb, Oshikoto, Namibia.

Its hardness is 3.5–4 (calcite is 3); a steel knife scratches it. Like calcite it fizzes in dilute acid. Its specific gravity is about 2.9, a little heavier than calcite (2.7). Calcite has perfect cleavage in three directions; aragonite's cleavage is poor (distinct in one direction only). Its crystals are mostly long, needle-like or prismatic; columns that look hexagonal because of twinning are common. Banded stones sold as “onyx” or “onyx marble”: according to Britannica, “onyx marble” consists of zones of calcite or aragonite; it is neither true onyx (banded chalcedony, of the quartz family) nor true marble. In some parts of the world this stone is aragonite; the stone worked as “onyx” in Türkiye is, according to the Küre Encyclopedia, a calcite-based travertine.

On the “street to diamond” value ladder it sits on step 3 of nine: Building and industrial stone. Aragonite is a common mineral. In collecting, large, well-formed or twinned crystals, branching “flos ferri” clusters and specimens from famous places such as Molina de Aragón, Tsumeb or the Erzberg are sought after. As an ornamental material its value depends on the colour and pattern of the banded stone, its translucency and the size of the piece; because it is soft and attacked by acid it is used more for vases and ornaments than for jewellery.

According to a caption in Kunz's book, an aragonite pendant was used for votive (religious offering) purposes in Armenia. Some news websites in Türkiye write that aragonite “grounds” the body by regulating the root chakra, reduces anger, stress and tension, and strengthens sociability and communication. The same articles claim aragonite keeps bones and joints healthy, maintains the body's calcium balance, strengthens immunity, and helps colds, pain and the healing of small wounds and burns. There is no scientific evidence that aragonite affects bones, immunity, pain or wounds. Carrying a stone does not contribute to the calcium balance; calcium comes from food. Fractures, wounds and burns should be treated by a doctor. Aragonite is a soft carbonate that fizzes in acid; leaving it in acidic liquids such as vinegar or lemon juice ruins its surface. This section reports beliefs; it is not health information. There is no scientific evidence for these claims and no stone replaces medical treatment.