there are some variations in the structure of the rock based on where it's from, mostly based on geologic and volcanic activity, and even smaller variations in the type of microfauna you'll get with them
yeah... how much was that priced at? if it's anything over say 4 dollars a pound (2-3 being more reasonable) don't bother
most likely what it was was somebody who tore down a tank gave/sold the rocl to the store and the store let it dry out to sell
it's nowhere near live anymore