It is the fact that calcite is readily dissolved that these formations occur. Overlying limestones or marbles are dissolved away by years and years of slightly acidic ground water to percolate into the caverns below. In fact the caverns themselves may have been the result of water dissolving away the calcite rich rock. As the calcite enriched water enters a relatively dry cavern, the water starts to evaporate and thus precipitate the calcite. The resulting accumulations of calcite are generally extremely pure and are colored if at all, by very small amounts of iron or other impurities.
Mexican onyx is a variety of calcite that is used extensively for ornamental purposes.
It is carved into figurines and is so popular that almost every child in the USA owns a small onyx animal or two. Carvings such as vases, bookends, plates, eggs, obilisks, pyramids and statues are all popular. To avoid confusion it is best to refer to it as Mexican Onyx. Mexican onyx is banded with multiple orange, yellow, red, tan, brown and white colors that have marble-like texture. Originally discovered and named after Eskifjord, Iceland where the calcite is found in basalt cavities.