If you follow the blog, you probably remember a previous post on never having enough holey rock. What that post didn't include was any tips on selecting the "right" pieces of rock. Of course, such decisions revolve around what you want your tank to look like, what you like in holey rock, etc.
However, there are some practical reasons to be selective beyond the appearance you're striving for in your tank. Here are a few questions you might want to answer:
What size are the cichlids you are keeping?
Do you have at least a pair of rock dwelling cichlids?
What kind of substrate do you have, if any?
In my experience, holey rock comes in two basic types - course and smooth. You'll easily notice the difference if you see enough of it. The former variety tends to have more holes or pits. It's also rather jagged throughout, which can pose a problem. The good news is that no two pieces are shaped the same. Just stacking a few together will almost always create numerous crevices, caves, and pass throughs.
Since I only keep dwarf species of cichlids, holey rock with built-in holes are ideal (see images 2a and 3b below). I don't have to always strategically stack the rock to create caves. In my experience, the smooth variety of rock tends to have the larger holes. I also use sand substrate, so I look for rock that are long and somewhat thin with "arches" on one side (see image 2b). I also like flat pieces that I can easily stack on (see image 4). I use these as my base. Many African cichlids like to dig, especially mbuna. So I like to take the rock with arches and put the arch side down on the sand. This creates natural caves, and the sand can be dug out a little making the caves larger. I will intersperse some course rock on top of the smooth rock, where fry have numerous little holes to seek refuge.
The Cichlid Room Companion
Tropical Fish Hobbyist
American Cichlid Assoc.
African Cichlid Hub