There are different types of base rock, the basic three are:
The most prefered is aragonite, which is calcium carbonate based rock. This was rock that was at one time in the ocean even if it was last year or hundreds of years ago. It is usually white to tan and a very hard to medium hard rock which coralline grows very well on.
Tufa Rock is also a choice that is normally tan to brown. Tufa rock was formed by geothermal waters rich in minerals though most of it is reef safe some may not be. It ranges from hard to soft and coralline grows very well on it.
Lava this is a rock that needs to be tested before addition to an aquarium, it can contain many minerals in it. It is black to brown to red in color and can be very hard to almost glass like, coralline can grow well on these rocks. But I would use with caution.
Thomas