I have had 3 cm. pieces of marble in this machine, not as large as the bed size but around 10 x 12.
The support for the glowforge is two wide ridges that run the full width from side to side in the front and back, so the bed is fully supported… as long as it is sitting flat.
Within the parameters of possible thickness and x/y dimensions, I don’t think there is any danger of harming the machine. As Tom said place the stone, don’t drop it. I would also take his advice and protect the bottom from scratches.
This is just my opinion, but the machine is well made, and you would have to be pretty rough to damage it. Just be sure not to drop it on the head or gantry! 