Dwarf Bread

"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.