Rocks That Alter Ph ?

The rocks that increase pH also increase hardness. It is usually rocks made from calcium carbonate that do this as they slowly dissolve putting both calcium (raises GH, which affects the pH) and carbonate (raises KH) into the water. Shells and coral are also made from calcium carbonate and do the same. The effect is more pronounced with low pH as calcium carbonate dissolves faster in acidic water.
Pebbles would be better, not just the small inch across ones but also large ones like tennis balls. Pebbles, as you know, are also smooth not rough which is important for bettas.
If you do choose a piece of rock from a shop, test it with vinegar. Just a few drops on the surface of the rock is fine. Watch for bubbles, even a few. If there are any, the rock is calcium carbonate, if there isn't a single bubble it's not and is safe to use.
The other way to test it is to put the rock in a tub of water, and also put another tub/glass of just water to stand next to it. Test the pH of both lots of water next day and again after a week, and compare the 'with rock' water to the 'without rock' water. If they are the same, it is safe to use the rock.