You're not doing microbiologic culture here; hydroponics media don't really have to be sterilized, just clean.
Plants grow in DIRT for pete's sake! Its not like dirt is sterile.
If you're worried about this, boiling in water ought to be fine.
Hydroton/expanded clay pellets are popular because they are light, offer good water absorption, easy to re-use, and easy to work with.
As mentioned, ordinary gravel, lava rocks, perlite (or mixes of perlite and vermiculite) and coco coir are all passable substitutes for hydroton/clay pellets.
Glass marbles probably could work, but they're suboptimal because they're heavy, and not very porous; they won't retain much moisture.
Other alternatives?
Crushed brick would probably work well.
Some plants actually grow great in pure sand, though I don't think this is ideal for cannabis.
I've actually seen or heard of hydroponic grows done using chopped up cardboard, shredded polyester cloth, styrofoam packing peanuts, sawdust/wood chips, and even Legos! (The nooks in the legos retain water). Some of these things aren't reclaimable, nor typically used for cannabis, but they do give you food for thought.
I seem to remember someone talking about fantastic results using cheap shredded rubber mulch as a substrate. That's probably a reasonable thing to try.
Basically, the only point of the medium is to give the roots something to grab onto to hold the plant in place, and to retain moisture. Pretty much any pH-neutral inert substance that allows free air flow and retains some moisture could work, though obviously, some mediums are going to work better than others.
