The most common stone used for statuary carving generally these fall into three categories: alabaster, limestone and marble. These are all calcium based. Alabaster is the softest of these (2 to 2.5 hardness) composed of gypsum or chemically, calcium sulphate. It can be carved with wood carving chisels and rasps, although stone chisels are faster. They should be sanded to at least 600 grit to get a shine and they make beautiful colorful indoor sculptures.
Limestone is composed of calcium carbonate (old bones) that got cemented into a sedimentary stone. It is a little bit harder (2.5 to 3 hardness) but still easy to carve although there is great variability in limestone hardness. There are many colors of limestone from gray to tan to yellow to white. Limestone can be polished but does not achieve the crystalline finish of alabaster or marble. It can be left outdoors.
Marble is limestone that has been heated and compressed – metamorphosed – into that crystalline carving stone. Its hardness varies quite a bit (3 to 4.5) with some of those in the Southwest U.S. being very hard (5). Like alabaster, marbles come in a wide range of colors but can be displayed outdoors. In the marble family are travertines that are formed with holes in them and onyx marbles (as opposed to silicate onyx) that have different carving characteristics.
ALABASTER: MADE IN AMERICA
The best American alabasters are from Utah and New Mexico. They are not hard yet very consistent for carving and come in a great variety of colors.
ALABASTRO: FABBRICATO IN ITALIA
Italian alabasters are one of the tightest allowing for very detailed designs. They are mostly translucent. The white is called “Ice” and finishes like frosted glass. There are also sky blue and champagne (called Agata) versions of this fine alabaster.
Nature provides us with a huge pallet of stone colors and patterns. Considerations beyond stone hardness are brittleness, the complexity of the pattern and the ability to take a sharp line or thin section. These may be liabilities OR they may be assets.