White, creamy limestone has been a staple of Pacific Northwest sculptors since 1992 when 6 tons of Utah oolitic found its way to the Northwest Stone Sculptors 5th annual carving symposium.
It owes its continued popularity (10 tons at the 1993 symposium and many more tons subsequently) to its ease of carving, low cost and wonderful finished demeanor. It is indeed a “poor man’s marble,” both
genetically and artistically. With sweeping curves and shapes, limestone casts beautiful shadows and interesting lines.
Limestone is a rock that contains 50 percent or more calcite (calcium carbonate, CaCO3) and dolomite (magnesium calcium carbonate, MgCaCO3), of which calcite is dominant. It is the sedimentary parent of the metamorphic rock, marble. In very simple terms, it is a rock from which lime can be produced. Although it is defined chemically, the calcium carbonate can be formed in more than one way:
- agglomeration of many smaller carbonate particles
- chemical precipitation
- biological growth
The most commonly utilized limestone is oolitic or compact limestone that is formed from marine oolites or ooliths. Ooliths are small round or oval bodies, 0.25 to 2 millimeters in diameter that form concentric circles and/or radii of calcium carbonate around a nucleus by chemical precipitation. The nucleus may be a shell fragment, a small piece of algae, or a quartz sand particle. It is important that the oolith be continuously wave or current agitated, so concentric growth can continue.
The ooliths eventually become heavy enough to settle to the ocean floor, accumulate in thick layers, and over geologic time, through the pressure of the overburden, form limestone that is relatively compact and uniform. Just imagine; all those oolites huddled together at the bottom of the ocean waiting to be discovered by a stone supplier. Their growth is similar to the growth process of a hail stone. Another type of precipitate is travertine, in which calcium carbonate precipitates out of a cave of spring water and forms interestingly shaped and colored rock. Among others, this is the type of rock that forms stalagmites and stalactites in caverns. This rock typically contains voids, because of the irregular way in which it is deposited. Biologically formed limestone includes tufa, which are small calcium carbonate secreting fresh-water organisms, and reef-building corals. Both of these rocks are beautiful in their natural state, but are not considered to be carving stones.