Structures in a rock that give an indication of the direction of up and down are precious when rocks have been tilted or overturned. The oval body in this ancient altered basalt is an amygdule—a former bubble filled with minerals—and the geopetal structure in it is the set of fine mineral layers, which show the direction of gravity as surely as a spirit level does.
Geopetal structures are also common in limestone, where voids may be gradually filled with calcite or quartz as the rock sits underground over millions of years. Fossil shells and geodes are good places to look. When other indicators are sparse or ambiguous, finding a clear sign like this can be a big relief.
