[MENTION=380478]finnystar2992[/MENTION]; If placed gently, plenty of people before you have placed rock directly on the glass bottom of a tank and succeeded well, some of them even going bare-bottom.
It is not with out risk, however, as the rock does not contact the glass in a dispersed way. There are pressure spots where pointier features on the rock concentrate all weight into a small area of contact. This can be a problem but is not guaranteed to do so.
One thing many have done in the past is to custom fit and place a thin sheet of "starboard" material on the bottom of the tank, then place the rock and sand.
[MENTION=357162]Cordan[/MENTION]o also makes a great point with regard to ensuring the rock is contacting the bottom surface (whether acrylic, glass, or starboard) as opposed to just setting on the sand. This helps mitigate the risk of the rockscape shifting or tumbling down down the road when sand-sifting creatures cause the sand bed to move.