Hello Eileen - A very interesting stone indeed. I would say that it is a 'Float Stone' Although, I could be wrong, as I've not seen it in situ - These were placed just outside the furnace in a smelting house to catch and direct the molten tin into a 'Mould Stone' from whence an ingot of tin would be obtained - This is called 'White Tin' and 'Black Tin' is the crushed fine particles of cassiterite that is placed within the furnace along with charcoal, or peat in some cases.
These finds are rare in Devon, and I'm not to familiar with the Cornish artefacts. It might be worth contacting Colin Buck or Adam Sharpe of the Cornwall HES
Kindest regards Tin Miner