depends on the type of blackrock your selling. You have the mined up "a small piece of blackrock" which is very common these days due to the miners. You have "a large piece of blackrock" that comes in various weights from 5 to 25 stones and 4 different shapes. Lastly, you have the event dropped blackrock "a piece of blackrock" that comes in various large gem style shapes. The event dropped blackrock would be the higher value one considering most of it was turned in. During the Blackrock merchants days before they redesigned haven people were awarded high value items for turning it in.