Coin collectors are finicky, Mohja, and they shy away from any coin with the least little bit of damage. The bevel on your coin which makes it into a necklace produces such damage and, for that reason, your coin will have no value to collectors.
That does not mean your coin has no value. In fact, you have a gold sovereign or half sovereign from Great Britain. These fine looking coins are made of 0.2354 and 0.1177 troy ounces of gold respectively. At roughly $1400 US dollars per ounce these days, the value of the gold in your coin is more than 0.2354 x $900 = $330 or 0.1177 x $900 = $165. You can tell a sovereign from a half sovereign by measuring it. A sovereign measures 22 mm in diameter and a half sovereign only measures 18 mm.
Use a precious metal web site such as kitco.com to find the current value of gold. It changes every day. If you went to sell your piece to a gold dealer, he or she would charge a commission for buying it. You can also take your item to a jeweler for an in-person appraisal.
For pictures and more information on sovereigns and half sovereigns, click to this CoinQuest link.