Enchantment, 2W (3)
Uncommon
Whenever an opponent casts a creature spell, if Opal Acrolith is an enchantment, Opal Acrolith becomes a 2/4 Soldier creature.
{0}: Opal Acrolith becomes an enchantment.
Illus. Robh Ruppel

10/4/2004: When it turns into an enchantment, it is no longer a creature.
10/4/2004: Changes into a creature even if the spell is countered.
10/4/2004: When it turns into a creature, it is no longer an enchantment.
10/4/2004: It triggers when the spell is cast, which means it becomes a creature before that spell resolves.
4/1/2008: 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.
8/1/2008: A noncreature permanent that turns into a creature is subject to the "summoning sickness" rule: It can only attack, and its {T} abilities can only be activated, if its controller has continuously controlled that permanent since the beginning of his or her most recent turn.
Legal in Vintage (Type 1)
Legal in Legacy (Type 1.5)
Legal in Classic (MTGO)
Legal in Commander