Carbonate of lime forms the principal ingredient of limestone. A pure limestone should consist only of carbonate of lime. However, none of our natural stones are chemically pure, but all contain a greater or less amount of foreign material. To these impurities are due the beautiful and variegated coloring which makes limestone valuable as a building material.

Limestone occurs in stratified beds, and ordinarily is regarded as originating as a chemical deposit. It effervesces freely when an acid is applied; its texture is destroyed by fire; the fire drives off its carbonic acid and water, and forms quicklime. Limestone varies greatly in its physical properties. Some limestones are very durable, hard, and strong, while others are very soft and easily broken.

There are two principal classes of limestone - granular and compact. In each of these classes are found both marble and ordinary building stone. The granular stone is generally best for building purposes, and the finer-grained stones are usually better for either marble or fine cut-stone. The coarse-grained varieties often disintegrate rapidly when exposed to the weather. All varieties work freely, and can be obtained in blocks of any desired dimensions.