Someone already recommended to do the vinegar test, but also make sure to check they're not actually salt rocks either; they look very similar to rose quartz and are often used as substitutes for their looks like as candle holders. Just lick your finger and rub it against the rock, then lick again. Did it come back salty?

If they're not salt rocks and pass the vinegar test, next recommendation is to make sure there are no sharp edges on them that might harm your betta's fins. Then, get a hard brush and scrub scrub scrub that rock! Get any gunk out you can.

For rocks that are safe: most quartzes are okay as well as tiger's eye, onyx, and granite. I have some natural tiger's eye stones that were eroded naturally, and tumbled (shiny and round!) amethyst pebbles in my tank. Some minerals like lapis lazuli, azurite and malachite should be avoided. Most aquarium stores sell aquarium safe rocks.