A Loose Rock.
A Loose Rock is a type of container, often the first type discovered in a new game.
They are quite simply loose rocks that may have been used as secret storage locations for treasure by other people, or perhaps even the demon population.
Hollow Logs are another terrain object similar to Loose Rocks that may contain hidden loot, however while they are present in Act I, they cannot be searched until Act III.
A Hollow Log.