The Opals were a special set of enchantments in Urza's Saga and Urza's Legacy that turn into creatures when an opponent plays a creature. This brings new possibilities with cards like Replenish and Crystal Chimes. Opals are undercosted compared to equivalent creatures (and fit snugly into a WW structure), but the Opal player sacrifices some momentum and risks playing against a creatureless or creature light opponent. Finally, Opal Titan is the most attractive of the Opals for casual players.

The basic rulings concerning Opals are:
When an Opal turns into a creature, it no longer counts as an enchantment. [Urza's Saga Rule Page]
An Opal changes even if the creature is countered. [D'Angelo 99/05/01]
An Opal 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 98/10/05]
A creature spell is any "Creature" or "Artifact Creature" spell. [D'Angelo 99/05/01] Older cards of type Summon are also Creature spells.
 

Opal Caryatid
W
Common, Enchantment, Urza's Saga
When an opponent plays a creature spell, if ~this~ is an enchantment, ~this~ becomes a 2/2 Soldier creature. [Oracle 99/05/01]
Artist: Berry
Released: 10/1998
 

Opal Gargoyle
1W, 2/2
Common, Enchantment, Urza's Saga
When an opponent plays a creature spell, if ~this~ is an enchantment, ~this~ becomes a 2/2 Gargoyle creature with flying. [Oracle 99/05/01]
Artist: Kev Walker
Released: 10/1998
 

Opal Acrolith
2W
UnCommon, Enchantment, Urza's Saga
Errata: Whenever an opponent plays a creature spell, if ~this~ is an enchantment, ~this~ becomes a 2/4 Guardian creature. 0: ~this~ becomes an enchantment. [Oracle 99/05/01]
Artist: Robh Ruppel
Released: 10/1998
 

Opal Avenger
2W
Rare, Enchantment, Urza's Legacy
When you have 10 life or less, if ~this~ is an enchantment, ~this~ becomes a 3/5 Guardian creature. [Oracle 99/05/01]
Flavor Text: As the sun grew cold in the realm, the statue grew warm.
Artist: Edward P. Beard Jr.
Released: 2/1999
 

Opal Champion
2W
Common, Enchantment, Urza's Legacy
Errata: When an opponent plays a creature spell, if ~this~ is an enchantment, ~this~ becomes a 3/3 Knight creature with first strike. [Oracle 99/05/01]
Artist: Edward P. Beard Jr.
Released: 2/1999
 

Opal Titan
2WW
Rare, Enchantment, Urza's Saga
When one of your opponents successfully casts a creature spell, if Opal Titan is an enchantment, Opal Titan becomes a 4/4 creature with protection from each of that spell's colors and that counts as a Giant.
Artist: Paolo Parente
Released: 10/1998
 

Opal Archangel
4W
Rare, Enchantment, Urza's Saga
Errata: When an opponent plays a creature spell, if ~this~ is an enchantment, ~this~ becomes a 5/5 creature with flying that counts as an Angel. Attacking does not cause Opal Archangel to tap.
Artist: Jeff Miracola (23/350)
Released: 10/1998