Agate is a variety of quartz characterised by its concentrically banded appearance. Banded agate forms by the permeation and build-up of layers of silica in cavities within rocks. It is frequently found in basalts. The silica solution separates into layers that contain water and others that do not, resulting in the distinctive concentric bands.

Agates come in a large variety of colour combinations, almost all of which are a result of the inclusion of iron oxides in its chemical composition. Agate can range from opaque to transparent and has vitreous lustre. Several varieties were used as inlays on the Courtauld’s richly decorated Baroque frame, as seen in these images.

Whilst now commonly found worldwide, Germany was one of the best known historic sources of agate with over 50 quarries found in and around the famous lapidary (stone-cutting) workshop of Idar-Oberstein. Situated on either side of the fast-moving Nahe river, Idar-Oberstein quickly became one of the main stone-cutting centres in the industry, since the high-powered water provided enough free energy to turn the large sandstone grinding wheels that were up to 3.3m in diameter.

Agate has also been valued for its hardness as well as its decorative properties, with uses including polishing gold. Small hand tools with differently shaped agate tips have been used for burnishing gold leaf for centuries up until the present day.