DIY Rock backgrounds

Dear Bob,
I am in the process of setting up a large freshwater tank and would like to include extensive rock backdrop as well. Could you tell me how I can achieve this? How do I stack them (or glue them?) together to get the desired effect?
What glue is to be used? Here in India, I can obtain good quality marble, sandstone or slate. Which of these is safe to use?
I do look forward to your advice and help.
Thanking you, Arvind
>>
A few ways to go here... you might want to simply stack the pieces at this juncture, though a permanent sort of arrangement can be executed (very heavy... and permanent) by a few mechanisms... putty, resin.... Depending on the type of livestock, you might want to use marble (will make the water more hard and alkaline) if your livestock like the shift in water quality... or slate (more chemically inert)... by and large sedimentary rock (like sandstones) are not a good idea in captive systems... fall apart, grind down pumping mechanisms... make a mess...
Bob Fenner