inviolable, infrangible, splinterless, shatterproof, splinterproof, hard, breakable, hard-wearing, heavy-duty, adamant, more...

fragile, breakable

ceramic, titanium, carbon fiber, steel, aluminum, more...

hard to break,   impossible to break,   unable to be broken,   not breakable,   cannot be broken,   unable to break,   difficult to break,   cannot break,   can't break,   never breaking