any ideas if rocks are safe

I have some smooth rocks I found outside that I want to put in my betta fish tank. How can I wash them to ensure they are aquarium safe?

I'm planning on creating some rock formations in my 15 gallon aquarium! After looking online at some white pebbles I went to a local garden centre and bought some Pearl white pebbles without giving it a thought if they were safe! They glitter and have hints of brown and grey running through them...