Ummm, ahhh, it's geological shorthand for a rock containing the minerals quartz, feldspar, and hornblende or quartz, feldspar, and biotite. The quartz and feldspar are light-colored and the hornblende and biotite are black, so, if they're in a rock together, they give it a kind of salt-and-pepper look. Kind of like the Highland Granite tartan. More than you wanted to know, no doubt!!

Barb