Agate is a striped chalcedony quartz that forms in layers and in a wide variety of colors and textures. An individual agate forms by filling a cavity in a "host" rock. As a result, agate often is found as a round nodule, with concentric bands. In ancient times it was said to quench thirst and guard against fever, and agate bowls were very popular in historical Europe.
Citrine is quartz generally occurring in a golden color. Citrine occurs naturally in Scotland and in the Ural Mountains of Russia. Much citrine available today is actually smoky quartz or amethyst that has been heat-treated to achieve its characteristic color.