Siderite is an iron carbonate mineral - FeCO3. It is moderately common in hydrothermal veins, some concretions, and in some Precambrian-aged banded iron formations (BIFs). Siderite is usually light brown to dark brown to reddish brown in color. It tends to form distinctive rhombohedral crystals with curved crystal faces (see first pic below; dolomite crystals do this as well). Siderite also has three cleavage planes not at right angles (rhombohedral cleavage). Thanks to its iron content, siderite is moderately heavy for its size.
Siderite (field of view ~2.9 cm across) from the Vekol Mine near Casa Grande, Arizona, USA.
Siderite (3.4 cm across)
Siderite from Dalnegorsk, Maritime Province, eastern Russia.
(Wayne State University collection, Detroit, Michigan, USA)