Question on how to make fake rock

[Seahorse Lover 20]

Just wondering if anyone knows how to make fake rock for aquariums that wont leach out harmful materials.
Also want it to be real looking. Like dead coral. Well any ideas throw them out me please.

 

[Jake337]

I made some DIY live rock from Portland cement. Looks pretty real for me. Just go on the DIY thread and search diy live rock.