If you can, get it checked.
There are many alluvial stones, generelly those who are quite hard.
Just by 'look' you can have a hard to ID stones which have lost their natural shape. (E.g rounded)
Get specific gravity if you can.
A local school, if they have a lab should be able to help. (a scale is needed)
Then you:
Notice dry weight
and submerge it in water and notice wet weight
Then you use this formula:
dry weight/ (dry weight - wet weight) = SG (Specific gravity)