"No one ever went hungry when they had some dwarf bread to avoid. You only had to look at it for a moment, and instantly you could think of dozens of things you'd rather eat. Your boots, for example. Mountains. Raw sheep. Your own foot."
—Terry Pratchett (Witches Abroad)

Dwarf Bread comes from Terry Pratchett's Discworld series. It is rock-hard, and, in fact, contains rocks such as gravel and stone. Other ingredients may include brick-dust, sawdust, and wood shavings.