Opal Acrolith

• WWizards of the Coast2004-10-04
  Official Ruling
  It triggers when the spell is cast, which means it becomes a creature before that spell resolves.
• WWizards of the Coast2004-10-04
  Official Ruling
  When it turns into a creature, it is no longer an enchantment.
• WWizards of the Coast2004-10-04
  Official Ruling
  When it turns into an enchantment, it is no longer a creature.
• WWizards of the Coast2004-10-04
  Official Ruling
  Changes into a creature even if the spell is countered.
• WWizards of the Coast2008-04-01
  Official Ruling
  A "creature spell" is any spell with the type Creature, even if it has other types such as Artifact or Enchantment. Older cards of type Summon are also Creature spells.
• 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