Rule of Thumb...??

Hello again!

My question this time concerns the "Root over Rock" planting. So, I was wondering what should one inexperienced have in mind when the issue of choosing the right (most suitable) rock comes fore. What parameters should one take into consideration? Is there some kind of a rule of thumb which governs this style of bonsai?
Of course the purpose of this question is absolutely informative, encyclopaedic!
I do not mean in any case that I have in mind attempting a root over rock.... BUT, in the course of time who knows? Maybe I will have managed to get along and maybe I will try something advanced....

Thank you!

1. A weathered, natural looking rock, not some broken-off piece with sharp edges.
2. The kind of rock that might be found where he tree you intend to plant over it might also grow.
3. A rock with a higher vertical measurement than horizontal.
4. Something with natural "grooves" down which roots may be guided.

Willing to respond and inform, as always!
Thank you very much, Jim!