If their mother was raising them outside they would use sand and do fine. When they got a little bigger, they would use sand and small stones. If it is good enough for their Mommy, it is good enough for me.
The hardness of the sand will depend on your native rocks. Most grit you buy is made of granite, which is very hard. If you native rock is softer, it just means that the grit will get ground up a little faster in their gizzard, pass through their system, and they'll need to replenish.
One thing I would look out for. If it is beach sand, it might be salty. A quick rinse would take care of that and it does not take sand long to dry.
Good Luck!!