Marble to place speakers on?

I find this thread interesting.For a couple of years I have considered this as I have an old,crack infested wooden floor that makes spiking the speakers through the carpet/pad difficult.I never seem to be able to get them solidly spiked.My concern is the thickness of the marble and the subsequent raising of the tweeters way above ear level.The obvious remedy is to have the spikes out further in the back of the speakers.Please be kind to me in the follow up to this "obvious" remedy,but is this correct? Also,is this the best/only way?

Where do you guys get this marble? Just 3 days ago I put some ceramic tile 1/4"X 12"X 18" under my speakers.It was very cheap ($2ea.) and it was all I could find that would fit.I have my speakers spikes almost all the way into the speakers.They are out about 1/4" I hear no ringing or anything else other than deeper and tighter bass.I still would like to try some marble if I can find it.