Porfido Nero "Porissima Specie", Corsi. Bought at Thermae of Caracalla

Flat stone sample of porfido nero. Three of the four sides appear to be cut; two are parallel to each other and one runs diagonally between them. The fourth side is uneven. This may have been a wall or floor tile, since the thickness is an even 1 cm throughout. The majority of the stone is black with vivid green inclusions. Both sides of the stone are highly polished and flat.