White platy Calcite. This is a variety of Calcite called "argentine", which alludes to its silvery appearance allegedly seen in the miner's eye. Sorry, I don't see it either. The Calcite is composed of platy, undulating, crystals giving it a look of silver. The specimen originated from the Philadelphia collection break-up several years ago (note old ink label in the prov. tab). You will also see an example of "argentine" from Cornwall on Mindat. Even the English were fooled. Historic significance.