Hi,
Looking at the photo,they look like Sandstone which should not alter your PH. It is worth checking though, and the way to do it is to add some weak acid and if there is any fizzing or bubbles this shows that the rock contains carbonate and will make the water more alkali,in other words raise the PH. The fizzing is caused by Carbon Dioxide being given off.
The acid used is better the stronger you can obtain,within reason! Usually white Vinegar will work but if you can get something stronger that would be better,your local fish shop may be able to test it for you.
Did you purchase the rocks from a fish shop,or collect them yourself?
I also would recommend scrubbing and boiling the rocks for a few minutes in boiling water to kill any nasties off,some people say you can place them in bleach but you need to make sure you get it all off after, and as Sandstone can be a bit porous it may soak inside it and then slowly come out into the aquarium water.You could also put the rocks in some water in a bucket, test the PH before putting the rocks in,then keep checking the PH over a week or two and see if it alters,but this will take time.
I don't know anything about keeping Crayfish but I hope this helps.