Yes, names of rocks are generally proper nouns. Rocks consist of minerals, and because of the scientific naming process, their names tend to be specific and therefore, proper nouns. For example, Granite is the name of an igneous rock which consists of quartz, feldspar, and mica.
Rocks often only have one name associated with them, so they are usually treated as proper nouns. Sometimes their names are descriptive, like sedimentary rocks called conglomerate and sandstone, while other rocks have been given unusual names like dunite.
It is important to note, however, that not all geologic features or objects have proper nouns. For example, large collections of sediment are known as an alluvial fan, and a lava tube is an underground passageway formed by a lava flow.
For more information about rocks and their names, check out this helpful article from the US Geological Survey: https://www.usgs.gov/special-topic/rocks-and-minerals
