Agate is a naturally occurring variety of chalcedony, a microcrystalline form of quartz, distinguished by its characteristic bands, layers and variations of colour. Valued since antiquity for its beauty, durability and ability to take a fine polish, agate has been used for thousands of years in jewellery, carved objects, seals and decorative works and later became an important material within European furniture and the decorative arts.

Agate forms when silica-rich material is deposited gradually within cavities in volcanic or other rocks. Successive layers of microscopic quartz crystals build up over time, creating the distinctive bands, patterns and variations in translucency for which the stone is known.

Its natural colouring is extraordinarily varied. Agates can contain combinations of white, grey, brown, red, orange, yellow, blue and black, although some historic and modern examples have also been artificially dyed to intensify or alter their appearance.