Any old rock can be used as a tool. It seems to me though, that the spirit of the term "tool" in this forums context requires that it be purposely shaped. Thus showing percussion marks or scrapes or wear from heavy use. These should be reasonably obvious. You'd have a hard time convincing an archaeologist that this rock is a tool. At least from the picture.
It has a wedge shape and appears to have impact marks all over except for the fine line on the ridge. It also has a perfect fit in hand. I can usually decide in the field or even when I do a second clean up.
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'!