Quartz is one of the most abundant substances in the earth’s crust, but mankind has treasured it for its beauty in a variety of forms since ancient times. Some of its forms, such as colourless rock crystal, amethyst and citrine, are single, transparent crystals of quartz. However, quartz can also grow as millions of interlocking microscopic crystals (‘cryptocrystalline’ structure), and in this state it is known as chalcedony. Chalcedony can take a number of forms as well, but perhaps the most famous is agate. Agate is the name of any form of chalcedony that is exposed to different colouring agents during its growth and displays a banded, layered structure as a result. Agate is therefore a beautiful and tremendously varied stone, renowned for its intricate patterning. This has been put to dramatic use for millennia, cut en cabochon or polished into beads to display their striped patterns, or ingeniously carved in cameos, with one layer carved in relief to stand out against a contrasting background. A hard, durable and surprisingly affordable stone, agate is suitable for almost any type of jewel, and its variations in pattern ensure that no two examples are alike.