A noncreature permanent that turns into a creature can attack, and its {T} 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.
After the triggered ability resolves, Opal Guardian is no longer an enchantment.
Guardian's effect doesn't wear off.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Christopher Rush
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A face-down creature spell will cause Opal Guardian's ability to trigger (if it's an enchantment).
