You're likely thinking about lichen! These fascinating organisms are a symbiotic partnership between algae and fungi. They can be found growing on a variety of rocks, including:

* Granite: Often found in mountainous regions and along coastlines.

* Limestone: Common in areas with high calcium content, often forming unique patterns.

* Sandstone: Found in deserts and arid regions, often with bright colors.

* Basalt: Volcanic rocks that are rich in minerals, providing nutrients for lichens.

You can find lichens growing on rocks in:

* Forests: Especially in moist and shady areas.

* Mountains: At higher elevations, where conditions are harsh.

* Deserts: Lichens can survive in extreme dryness.

* Coastal regions: Often found on rocks exposed to the ocean spray.

Remember that lichens are very sensitive to pollution and can be used as indicators of air quality.

If you're interested in finding lichens, be sure to look in places with plenty of moisture, sunlight, and a suitable rock surface!