Thank you very much for your replies! Iīm not a coin collecter and I really appreciate your help. I was reading about this coin and it seems that the coins minted in London in 1917 are very rare and quite valuable. The one I have has no mint mark so it would indicate that was minted in London (the ones minted in Australia, Canada, etc, have a mintmark above the date). But like GDJMSP said here
http://www.cointalk.com/t68335/ it could be fake because itīs a very counterfeited coin.
Thatīs why, Fullmoonkid, I was asking that if the coin is made of gold is enough to determinate its authenticity. Or maybe the forgeries were also made of gold. For GDJMSPīs reply I assume itīs a possibility. In which case its only value is the gold value...I donīt know if Iīm making myself clear I donīt speak english very well!!
Thanks fretboard for your advise, Iīll try to weight it, I donīt know if itīs a half or full sovereign, in fact I donīt know anything about coins! I tried to post pictures but I couldnīt do it, Iīll try again later.
Thank you all very much!!