Quartzite is a compact fine-grained but distinctly granular rock , very hard , frequently brittle and often so divided by joints as to split in all directions into small angular, but more or less cuboidal, fragments. The colors are generally some shades of yellow, passing occasionally into red and at other times into green. When examined with a lens it may be seen to be made of grains, which appear some times as if they have been slightly fused together at their edges or surfaces, and sometimes as if imbedded in a purely siliceous cement. This cementation or semi-fusion of the grains shows at once that it was a sandstone which now altered and indurated by the action either of heat alone or of heat and water, the process is known as metamorphism.