This section is from the book "Spons' Mechanics' Own Book: A Manual For Handicraftsmen And Amateurs", by Edward Spon. Also available from Amazon: Spons' Mechanics' Own Book.
Granite generally contains more felspar than quartz, and more quartz than mica. The colour of the stone depends upon that of the predominating ingredient, felspar. An average granite may be expected to contain 2/5 to 3/5 of crystals of quartz or crystalline quartz; about the same, more or less, of felspar, also partly crystalline and chiefly in definite crystals; and 1/10 of mica. But the mica may form 2/10 or 3/10 and the quartz more, while the proportion of the felspar, as well as the particular composition of the felspar, both vary extremely. The durability depends upon the quantify of the quartz and the nature of the felspar. If the granite contains a large proportion of quartz, it will be hard to work; but, unless the felspar is of a bad description, it will weather well. The felspars that occur most commonly in granite are potash felspar (orthoclase) and a lime and soda felspar (oligoclase). Sometimes both these varieties are found in the same stone. Of the two, potash felspar is the more liable to decay. Mica is easily decomposed, and it is therefore a source of weakness. If the mica or felspar contain an excess of lime, iron, or soda, the granite is liable to decay.
The quantity of iron, either as oxide or in combination with sulphur, affects the durability of granite, as well as of all other stone. The iron can generally be seen with a good glass; a very short exposure to the air, especially if assisted in dry weather by artificial watering (better if 1 per cent. of nitric acid be added to the water), ought to expose this. The bright yellow pyrites crystallized in a cubical form appear to do little harm. The white radiated pyrites (marcasite), on the contrary, decompose quickly. Where the iron stains are large, uneven, and dark coloured, the stone may be rejected for outside work. When the discoloration is of a uniform light yellow, it is probable that little injury will be done to the stone in a moderate time, and unless appearance is a matter of great importance, such granite would not be rejected. In red granites, the discoloration from iron does not show so easily, but still sufficiently if bad enough to cause rejection. The quality of granite for building purposes depends upon its durability, and upon the size of the grains. The smaller these are, the better can the granite be worked, and the more evenly will it wear.
In using granite for ornamental purposes, the coarser-grained stones should be placed at a distance from the eye, the finer-grained stones where they can be easily inspected. Without attention to this point, very little better effect is produced than by a stone of uniform colour. Granite is quarried either by wedging or by blasting. The former process is generally reserved for large blocks, and the latter for smaller pieces and road metal. It is better to have the blocks cut to the desired forms in the quarries; first because it is easier to square and dress the stone while it contains the moisture of the ground or "quarry-sap"; also because the local men, being accustomed to the stone, are able to dress it better and more economically, and part of the work can be clone by machinery. Moreover, the bulk of the stones being reduced by dressing, the cost of carriage is saved, without much danger of injuring the arrises in transit, as the stone is very hard. It is used chiefly for heavy engineering works, such as bridges, piers, docks, lighthouses, and breakwaters, where weight and durability are required. It is also used especially for parts of structures exposed to blows or continued wear, such as copings of docks, paving, etc. The harder varieties make capital road metal.
In a granite neighbourhood the stone is used for ordinary buildings; but it is generally too expensive in first cost, transport, and working, and is therefore reserved for ornamental features, such as polished columns, pilasters, heavy plinths, etc. The granular structure and extreme hardness of granite render it ill adapted for fine carving, and its surface is entirely destroyed by the effects of fire.