Try hard not to fall for the notion that if it 'fits well in hand' that it's a tool/ artifact. I have yet to see a tool that was designed or manufactured to fit well in the hand. However, like Claimjumper said, nature provides any number of rocks which can be used as tools. If they're not actually 'worked' or show actual signs of human use, ya can't call it an artifact. Keep lookin'!