Is it necessary for the substrate to be pea gravel or will decorative gem stones work the same?
Functionally pea gravel will work better than gem stones as substrate in your tank.
The main goal of substrate is to improve the quality of the water in your tank, this is done by the giving the beneficial bacteria in your tank a place to grow.
While beneficial bacteria can grow on either pea gravel or gem stones, the gem stones may be heavier and food may sink throw the stones to the bottom of the tank where the fish cannot get to. This means this food will just rot and cause the water quality to deteriorate more rapidly.
So the gem stones will work, but mean more frequent water changes that you would need to stay on top of in order for your Betta to stay healthy.
Hope this helps!