The Guinea was a coin of approx. 1/4 troy ounce of gold and originally worth £1. Gold price fluctuations vaired the value but in the 1816 coin act it was officially fixed at 21 shillings £1.05. Guinea was not an official name for the coin, but much of the gold used to produce the early coins came from Guinea in Africa and thus the name was coined. - the Guinea was replaced by the Sovereign.
Specifications vary by a small amount, but average Weight: 8.5 grams Diameter: 25–26 mm . Also minted were 5 Guinea and 2 Guinea coins