One of the four basic blocks (Dirt , Rock, Lava, Cave Background). This can be bought at the store from the Basic Splicing Kit in sets of ten. It is a seed where almost all seeds are spliced from, making it an important and valuable element. It can be found in all worlds, even in some Blasted worlds.
During the Valentines' festival, Rocks change to Heartstones which can be broke to get many Valentines' 'presents', such as Angel Wings et cetera. However, a world that had had its rocks changed to heartstone is irreversible.