Limestone is one of most versatile stones available. In its pure form it is hard enough to be used in almost any application, however it is relatively soft compared to other building stones such as marble.
Forms of limestone include chalk (a much softer version), and marl (a type of fertilizer). Travertine is also a limestone, though harder. Travertine is often found in stalactite and stalagmites that have formed in caves.
Limestone is chiefly composed of calcium carbonate. It is a sedimentary rock, which means that it is formed by collection of other minerals coming together to bond at a structural level. This most commonly occurs when lime builds up in oceans, then washes ashore and dries out.
It is white in its purest form; however impurities that become stuck in structure during formation cause colorful streaks and shades to appear in its surface. There are literally an unlimited number of colors limestone can come in due to variety of impurities and nature of their relationship to forming stone. Iron will generally cause colors in limestone to shift to red, or yellow, while carbon will shift colors more towards grey or black. The erratic nature of way impurities join with limestone, means that each piece of limestone is unique. Despite these wide variations Limestone can be easily categorized into several subgroups.