Granite is the most valuable stone where strength is required, its crushing strength averaging about 20,000 lb. per sq. in. Owing to its hardness, it is very costly to dress, and its use is limited to the most expensive kinds of buildings. Granite being very dense and compact, absorbs but little water, and hence is valuable in damp situations. Exposed to fire, it disintegrates at a temperature of from 900° to 1,000°F., being less durable in this respect than fine-grained compact sandstones. The average weight of granite is about 167 lb. per cu. ft.

Limestone is a very common building stone, and. when compact, is very durable. It is usually quite absorptive, and becomes dirty quickly; while under intense heat, it is converted into lime. Limestone must be well seasoned before use, to get rid of the quarry water. The strength of limestone varies from 7,000 to 25,000 lb. per sq. in., the average being about 15,000 lb. The weight of limestone is about 155 to 160 lb. per cu. ft.

Sandstone is, in general, an excellent building stone, capable of resisting great heat, and the better kinds absorb only small quantities of water. The dark-brown, flinty sandstones retain their color very well, ranking better than granite. A stone containing much pyrites becomes unevenly discolored, due to formation of rust, and hence the stone should be carefully examined in this respect. The average strength of sandstones is about 11,000 1b. per sq. in., varying from 4,000 to 17,000 lb. The weight of sandstone is about 140 lb. per cu. ft.

The densest and strongest stones are generally the most durable. A fresh fracture, when examined under a magnifying glass, should be clear and bright, showing well-cemented particles. When a good stone is tapped with a hammer, it gives out a ringing sound. The absorptive quality of a stone may be tested by noting the increase in weight after soaking in water for 24 hours. One that increases 5 per cent, or more should not be used. For ordinary building purposes, tests of crushing strength are unnecessary, as if stone is of good quality the strength is generally very much in excess of any probable loads.