sometimes you also want to get the internal vibrations out, and then suspend 'that' contraption as well aa
My guess is that each component will react differently to difft materials. A lot depends on your floor, your speakers, the room, listening habits, etc, etc
For my system, i stay with the materials which many musical instruments are made out of: brass & tonewood. they seem to sound the best in particular applications.
But, this tweakier side of the hobby has a lot of variables.
Check the archives here and at audioasylum in the Tweaker's Asylum.
glass and aluminum? sounds like 2 hard materials...doubt they'd do well to isolate. Couple maybe, not isolate though.
Often, you'll find that most generic spikes are simply to hold components steady, or to get them off the carpet...they aren't engineered for any other purpose. to my ears these types of spikes usually sound bad.