Early tin streamer worked their tin ore on site. They would crush the tin-baring gravel in a block of granite hollowed out and used as a mortar, in much the same way as a mortar and pestle are used in the kitchens.
Occassionally you can still find discarded mortarstones in old tin-streaming areas. The best places to find them are nearly always near to an old tinners hut within a tin-streaming valley.
This example was found at Plym Steps tucked into the hillside by an old tinners hut.