any of various hard colored rocks (especially rocks consisting of chert or basalt)
source: WordNet 3.0
A provincial name given in England to basaltic rocks, and applied by miners to other kind of dark- colored unstratified rocks which resist the point of the pick. -- for example, to masses of chert. Whin-dikes, and whin-sills, are names sometimes given to veins or beds of basalt.