Agate is a form of chalcedony (quartz, silicon dioxide) called Cryptocrystalline typically filling cracks and gas cavities in ancient lava flows; Scotland's agates are from the Old Red Sandstone age (around 400 million years old).

In its purest form agate has little or no colour, usually grey or white, but because its microscopically fine fibrous crystal structure is porous, it takes on colours from mineralised ground waters circulating though rocks in active volcanic areas. The result is the distinctive banding (or moss or tree-like structures) and the huge variety of colours separating them from other kinds of chalcedony. Although the exact origins of the banding are still unclear most concentrically banded agates grow from the outside inwards, filling in the gas-bubble as they grow. The nature of the formation of agates means that no two will ever be alike and though widely available, there are forms of agate exceedingly rare and quite expensive to acquire.

Used for ornamentation and physical healing amulets dating back to Babylon, their medicinal use continued through Ancient Greek and Egyptian civilizations, spreading throughout Africa and the Middle East and into Russia, the agate has found favour in civilisations across the globe throughout the ages. Folklore mentions the use of agates as early as the 8th century while astrologers of the Middle Ages wore engraved signet rings as amulets. During the 1700s it was believed that the stone embodied the power to banish evil spirits and nightmares and to endow their owners with love, strength, courage, security and to provide comfort, dispel anger, help to balance the ego and support fidelity.

Used on a small scale for the manufacture of jewellery for at least two hundred years, agate is one of Scotland's best-known semi-precious stones. Cut into thin pieces it is nearly transparent but it less brittle than normal quartz, which helps to explain that stones found in cufflinks, bracelets and brooches made during the Victorian Era (140 years of age) show so little wear.

Antique Scottish Agate jewellery from Thistle & Broom’s private collection