Agates mostly occur as nodules in volcanic rocks or waters containing silica in solution possibly derived from the decomposition of silicates in the lava. The siliceous matter is deposited in regular layers resulting in the banded or striped formation.
Amethyst is a purple variety of Quartz often used as an ornament. It occurs in color from a light, slightly pinkish violet to a deep grape purple. Upon exposure to heat, Amethyst generally becomes yellow, which some refer to as Citrine or “burnt” Amethyst. It is widely distributed as a mineral, but fine, clear specimens that are suitable for cutting as ornamental stones.