Enchantment, 2W (3) Whenever an opponent plays a creature spell, if Opal Acrolith is an enchantment, Opal Acrolith becomes a 2/4 Soldier creature. {0}: Opal Acrolith becomes an enchantment.1
Illus. Robh Ruppel
When it turns into a creature, it no longer counts as an enchantment. [Urza's Saga Rule Page]
When it turns into an enchantment, it no longer counts as a creature. [Urza's Saga Rule Page]
Changes into a creature even if the spell is countered. [D'Angelo 1999/05/01]
It becomes a creature when the spell is announced, which is before that spell resolves. A Cloudchaser Eagle cannot be used to destroy this card, since it will no longer be an enchantment when the Eagle comes into play. [Urza's FAQ 1998/10/05]
A creature spell is any "Creature" or "Artifact Creature" spell. [D'Angelo 1999/05/01] Older cards of type Summon are also Creature spells.
Note - This card animated with creature type "Guardian" until Oracle 2007/10/01, when it became "Soldier".
Urza's Saga
(Uncommon)