The new description must contain a keyword of the item of origin. The item must be type miscellaneous, trash, or treasure, if I recall.

Keep in mind, decorate is for aesthetic purposes only, not gear. If I recall, decorate removes any wear properties (unlike sewing). So, if you have "a silver crown" that is item type treasure that can be worn on the head, you can decorate it, but the item will no longer be wearable (I think). Heralds with the "sewing" talent can alter wearable items to a limited extent (cloth, clothing, etc.)

The primary purpose for decorate is to create RP props and Inn decorations.