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Calcite is the most stable crystal structure of calcium carbonate, which is one of the most common minerals in the world. Calcite often exhibits rhombohedral crystals, as in this specimen. Like most carbonates, calcite dissolves when in contact of most acids. Calcite can be dissolved by groundwater or precipitated by groundwater, but that depends on several factors including water temperature and pH.
Chalcedony is a form of silica composed of quartz and moganite. Quartz and moganite have differently shaped crystals, and when they grow together, as they do with chalcedony, they form microsopic crystals that are even difficult to discern under magnification because of their size.
The calcite and chalcedony grew inside a basalt vug. This specimen from India measures 145x120x110mm, and weighs 1.59kg.