Your specimen is probably a piece of calcite.
Calcite is usually a white or colourless mineral with crystals that are
sometimes shaped like dog's teeth (hence the alternative name dog tooth spar).
However the crystals can take lots of different forms and the colour is very
variable.
Two particular features can help to identify calcite. When
rubbed against an unglazed piece of pottery it leaves a white streak and it
fizzes when dilute hydrochloric acid is dropped onto it.
Calcite is a very common mineral formed in a wide variety of geological situations
and can be found throughout the world. Some particularly well formed
crystals have been found in Derbyshire during mining. Calcite is used
extensively in the construction industry to produce cement.
