What you have there is a celt . If you found it I'd say it is authentic . Value is in the eye of the beholder, otherwise these are fairly common . the design to these tools didn't change throughout the ages so the only way to get a age on it is to find a spearhead, arrowhead or pottery from that site and age that. But even then you may not get an age as many sites were used over and over again throughout the years.
With all that said , it is a beautiful piece.