This obsidian lump comes from California's Napa Valley, where volcanic deposits help create the rich soil there. The outer rind shows signs of hydration from being buried in the soil for thousands of years. The thickness of this hydration rind is used for telling the age of obsidian, and hence the eruption that produced it.
Note the faint bands on the outer surface. These always remind me of taffy, and they result from mixing of the thick magma underground. The clean, black fractured surface shows why obsidian was valued by the native people for making arrowheads and other tools. Chunks of obsidian are found far from their place of origin because of prehistoric trading, and therefore they bear cultural as well as geologic information.
