Some of the vugs form from trees that were engulfed in a lava flow millions of years ago and have since withered away. Other vugs are just gas bubbles in the lava. Some vugs can be quite large. The crystals that form are usually light to medium in color and only colored at the tops of the crystals. Most clusters form with gray, white and blue agate and have a green exterior on the vugs.
Calcite sometimes is associated and inclusions of cacoxenite are common.
Clusters are druzy crusts that line the fissures formed in ancient metamorphic rocks. The crystals are usually colored throughout, unlike the Brazilian crystals, and form with a multicolored agate that often contains reds, yellows and oranges. Often amethyst- coated stalactites and other unusual formations occur inside these vugs. However, the interior color and clarity are excellent and polished slices and carvings as well as many gemstones are prized and admired. They are generally a brownish purple and patchy in color.