Agate stone Agate is a popular, semi-precious banded variety of chalcedony (a form of quartz) known for its intricate, colorful layers, often found in volcanic cavities and valued for jewelry and metaphysical properties like protection and balance. Formed from silica-rich fluids, agates come in many types (like moss agate, fire agate) and colors, distinguished by their distinctive banding or unique inclusions, making each stone unique.