Igneous Rocks:
Igneous rocks provide some of the highest quality of aggregate. The bulk chemical composition of igneous rocks is silicate, and depending or the type, the rocks contain varying amounts of quartz, feldspar, and dark ferromagnesian minerals. There are two major types of igneous rocks: Extrusive, fine grained, and intrusive, fine grained. The table below classifies the igneous rocks, based on their origin composition and colour.
| Colour | Light | Intermediate | Dark |
| Minerals | Quartz & Na Feldspar | NaCa Feldspar & Amphibole | Pyroxene, Ca Feldspar |
| Intrusive | Granite | Diorite | Gabbro, Diabase |
| Extrusive | Rhyolite | Andesite | Basalt |
Extrusive rocks are formed as result of volcanic activity; A special kind of extrusive rock is pumice, usually associated with rhyolite or andesite volcanism. Pumice is light-weight (SG <1), and used as lightweight aggregate. Cemented volcanic ash (ignimbrite) is sometime used as aggregate if well fused and hard.
Intrusive rocks are solidified from melt deep in the earth's crust. Most intrusive rocks are suitable as aggregate.
More extensive discussion of igneous rocks can be found here.
Although of high quality, some of the silicates in the igneous rocks may be reactive with the alkalis of the concrete, causing a destructive expansive reaction, known as ASR (alkali silicate reaction).
