The Firemaking ring is made by right-clicking the uncharged ring and selecting "Charge" within the Firemakingskill plot area of the Clan Citadel, which requires a tier 3 Citadel. The ring requires a Firemaking level of 40 to be charged. It gives 150% Firemaking experience until you have gained a certain amount of experience, determined by your level and the tier of the Firemaking plot, after which it crumbles to dust. A new uncharged ring can only be obtained after this one has been used up.
The clan ring works when it is either equipped or in your inventory. The experience bonus will stack with that of the ring of fire and/or flame gloves, giving you even more bonus experience. It is therefore more effective to have the clan ring in the inventory whilst the ring of fire is equipped. However, you would carry one log less.
The experience bonus seems to be far higher than other skill's respective rings - more than double the others. However, the output is lower than the penance horn's rate (200% exp as opposed to 150%), and since they do not stack, players may want to use their ring for another skill if they already have a charged penance horn.