It is a mafic rock,
containing magnesium and iron, often in form of olivine. Like other
mafic rocks, it is relatively low in silica content.
It is a medium grained igneous
rock, and can be dark
grey or black
with greenish shades in colour.
Minerals in this rock include olivine, plagioclase feldspar, pyroxene,
quartz and sometimes ilmenite, biotite, hornblende and magnetite.
By Oor Woolie via Flickr.com
It often forms in basaltic provinces as sills and dykes, (which are
By brewbooks via Flickr.com
Some of the most famous occurences are in New Jersey in The US
(the famous Palisades),
at Lake Superior in Canada, in north-east England (as well as the famous ancient stone circle Stonehenge!), and on the Tasmanian
island in Australia.