Umar bin Ya'la al-Thaqafi narrates from his father, and he from his grandfather (may Allah be pleased with him), that a man came to him (peace and blessings be upon him) wearing a large gold ring. The Prophet (peace and blessings be upon him) said: "Have you purified it (by paying its zakah)?" He said: O Messenger of Allah, what is its zakah? He said: When the man turned away, he (peace and blessings be upon him) said: "It is a huge ember."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7586