Chalcedony is a generic term for compact, micro- or cryptocrystalline quartz, either translucent or mostly opaque. It is white when pure but much chalcedony contains trace elements of microscopic inclusions of other minerals, giving a wide range of colors.

Opal is an amorphous hardened hydrous silica gel that forms at low temperatures from silica-bearing circulating waters. Opal lacks a crystal structure and specific composition and is therefore not a mineral, but a mineraloid.