Opal Guardian

• WWizards of the Coast2006-09-25
  Official Ruling
  After the triggered ability resolves, Opal Guardian is no longer an enchantment.
• WWizards of the Coast2006-09-25
  Official Ruling
  Guardian's effect doesn't wear off.
• WWizards of the Coast2006-09-25
  Official Ruling
  A face-down creature spell will cause Opal Guardian's ability to trigger (if it's an enchantment).
• WWizards of the Coast2008-08-01
  Official Ruling
  A noncreature permanent that turns into a creature can attack, and its abilities can be activated, only if its controller has continuously controlled that permanent since the beginning of their most recent turn. It doesn't matter how long the permanent has been a creature.

Rulings via Scryfall