here are things I consider quality frag plug material
white marble (never colored marble the color is from mineral deposits they can and will leach)
glazed ceramic tile, that is not mildew treated and has no glues or backings (you can remove glues and backings and clean them up manually but its a lot of work, the backings (webbing) is usually treated with mildew/antibacterial chemicals, believe it or not the cheaper the tile the better chance you have of it NOT containing milewicides, or antibacterial chemicals.
terra cotta, glazed or unglazed doesnt matter, fairly inert.
ceramic plugs
aragonite plugs
glass
plastics (acryllics, or most other polys like those little water holders for flowers) the water holders are frikken awesome for SPS and gorgonians you can use them with the lid or with out, filled with sand they are very stable when inserted into a sand bed. make sure they are new and have not been exposed to fertilizers or flower food/preservatives.
rubble rock
here is a wierd one but it works, silicone mixed with aragonite sand or crushed coral, make any shape size profile plug you want after 24hours perfectly safe.
DIY base rock style plugs, after cureing.
I feel like I am missing something but oh well.
UNSAFE: due to the potential to leach minerals/metals/chemicals
anything labled mildew resistant
natural stone (non aragonite rock), natural stone tile tile, many rocks have possible minerals you dont want in your tank, especially granite tile (leaches arsenic)
colored marble, minerals are the source of the color and some can be very harmfull while others will just promote nuisance algaes, White is the only safe marble.
slate, the composition makes it risky, though I have seen it used I would not reccomend it.
lava rock. nasty stuff, its man made and loaded with crap (even nitrates)
natural volcanic rock, though many reefs are based on, or around volcanic rock deposits the minerals dont concentrate in the ocean like they will in our tanks.
I think thats it for now If I think of anything else I will throw it on here, I hope this helps a bit.