Following are the properties of a good stone:
The appearance of a stone in relation to the design is of great importance from an architectural point of view. Appearance depends upon the colour and case with which the stone can be dressed, rubbed or polished. Deep colours, however, in sedimentary rocks are due to oxides of iron in the cementing material, which, upon exposure to the atmospheric influences, either fade away or disfigure and strain the surface on account of the rusting of iron. The stones which are to be used for face work should be attractive in appearance and should be of uniform colour and free from clay holes, spots of other colour, bands, etc. Stones with lighter shades are preferable, because even if they fade a little they will not show a striking difference and spoil the appearance.
It denotes the period in years for which a stone may stand practically unaltered after being used in construction. A variety of factors affect the durability of a stone. Of these, the mineral composition, texture and structure of rocks and their capacity to absorb moisture are very important.
Texture relates to the grains or particles composing the stone in the strata. A good building stone should have a compact fine crystalline structure, free from cavities, cracks or patches of soft or loose material.
5. Crushing strength:
It is also called compressive strength of a stone and is defined as the load per unit area at which a given stone starts cracking or failing. For a good structural stone the crushing strength should be greater than 100 N/mm^2.
7. Resistance to fire:
The fire resistance of a stone may be defined as its capacity to withstand very high temperature without disintegrating. For this requirement, the different materials constituting the composition of a stone should have different coefficients of expansion. In igneous rocks, like granite, free quartz is the most dangerous material as it undergoes a sudden expansion at less than 600 degree Celsius and flies into splinters; even timber is able to withstand a much higher temperature (about 800 degree Celsius). It then crumbles to powder and also increases in bulk. Sandstone with silicates as binding material are fire resistant. Clay stones have good fire resistance but are poor in strength and durability.