This is the easiest mineral to find on Guam. Limestone is made of calcite. It ranges from fine-grained and powdery to coarsely crystalline in rocks such as crystalline limestone and skarn. Calcite is so soft that it can be scratched by a penny. Our calcite is mostly white to colorless, but we also have pale yellow, pink, brown, and gray calcite.
Calcite fizzes in dilute hydrochloric acid.
A variety of microcrystalline quartz, this mineral is abundant around Guam's volcanic rocks. It results from the chemical weathering of basalt and other igneous rocks. Agate, a banded variety of microcrystalline quartz, is also common. This mineral is hard enough to scratch glass or the blade of a pocket knife. Chalcedony is colorless, and our agate is banded colorless and white.