From Halopedia, the Halo wiki
Granite (from Latin granum, meaning "grain") is a common and widely occurring type of intrusive, felsic, igneous rock. It has a medium to coarse texture, occasionally with some individual crystals larger than the groundmass forming a rock known as porphyry. Granites can be pink to dark grey or even black, depending on their chemistry and mineralogy. Outcrops of granite tend to form tors and rounded massifs. Granites sometimes occur in circular depressions surrounded by a range of hills, formed by the metamorphic aureole or hornfels. Granite is nearly always massive (lacking internal structures), hard and tough, and therefore it has gained widespread use as a construction stone.
The Highland Mountains on Reach were mountains of granite, and the planet Onyx also had numerous deposits of granite forming hills and mesas. Harvest's parliament building was constructed of granite. Certain Forerunner structures on Alpha Halo were also made using granite, with an intricate level of symbols embedded in their surfaces.