I too purchased my rock from local lawn and garden store. I chose hard smooth river stones for they are less likely to contain minerals that may influence the ph and hardness of the water in the aquarium.
rocks like limestone or coral,or rocks that appear pourus ,I don't use.
Flagstone and slate however are safe.
Would scrub all rocks with warm water and bleach solution and let air dry for 24 hours (turn em so bottom drys too) before placing them in my aquarium.
Would boil the wood and for very large pieces,,I soak them in tub of water for up to two months before using them and change the water a couple times a week.
Hope some of this helps.PS
I'm not sure bout rocks exploding either but why risk it?