Its very difficult to determine if something like that is an artifact or not without taking into account where it was found, if other artifacts were found, etc. Also, you can't go by "it fits nicely in your hand" because any and every rock can be turned in such a way as to fit nicely in your hand. If you found it in a field with obvious signs of indian activity, I'd say it's possible.. if you found it among river cobble or something, I'd lean towards no because that type of wear can occur naturally. It's always hard with a find like that... I'd hold on to it until you can figure it out.