The onyx stocking is a purely decorative item that looks like a metallic version of the kind of stocking you hang over the fireplace for Santa to fill.
How to Obtain
Onyx stockings are found only in bunkers in special holiday arctic biomes.
History/Trivia and Scam Alert
When the onyx boot accessory was first implemented, it was called a "stocking." With the introduction of holiday arctic biomes, the name of the accessory was changed to "onyx boot" so there would be no confusion between the accessory (boot) and the decorative seasonal item (stocking). Be aware that unscrupulous players will sometimes try to arrange trades based on the claim that the onyx stocking adds three points to the agility skill. The onyx stocking is NOT an accessory and will not increase your agility skill.