Onyx is a sedimentary rock, formed as stalactites and stalagmites in cave interiors. This formation method results in the cryptocrystalline construction of the rock fabric, and it is the size and uniformity of these crystals that contribute to the classic translucent property of most onyx varieties. While vulnerable to chemical and abrasive attack, the decorative appeal of onyx is perhaps unsurpassed by any other material.
Limestone deposits exist in all continents of the earth. Travertine is a type of limestone. It is actually the terrestrial (land) formed version of limestone, as opposed to the marine based formations of many other limestone varieties.These stones are some of the softer varieties of natural stone materials and can be used for intricately carved features and moldings. Limestone and travertine are a calcium carbonate base stone and are vulnerable to alteration by exposure to mild acids. The combination of acid sensitivity and absorption limit the number of varieties that are suitable for countertop applications.
Another form of limestone exists, which is dolomite that contains the mineral calcium-magnesium carbonate. Dolomites generally have higher densities, lower absorptions, greater compressive and bending strengths, and higher abrasion resistance than the calcium carbonate based limestones.