Use clear coloured plastics for the crystal to give the crystal its type (see below). Or simply use clear white plastic and place on a LED light box and shine the light through the crystal.
Crystals in Sword Art Online are considered to be one of the most convenient types of items and the closest thing to magic in Aincrad.
Most crystals can be obtained through certain NPC shops, except for the Corridor Crystal. Corridor crystals are the rarest of all crystals, and are only obtainable by trading with other players, or from killing monsters or obtaining them from a treasure chest.
Known Kinds of Crystal in Sword Art Online:
Antidote Crystal (Green) - cure/recover any status effects.
Healing Crystal (Peach Blossom) - completely heals the target.
Teleport Crystal (Blue) - transport the user to a chosen city's Teleport Gate plaza.
Corridor Crystal (Indigo) - a larger form of teleport crystal that opens a teleport gate to any location that the player has marked. Corridor crystals have a darker shade of blue than the teleport crystal.
Message Record Crystal (Light Sea Green when recording/playing-back, Teal when inactive) - record what someone has said.
Record Crystal - take in-game screenshots. Can print out via NPC shops later on.
Screen-Shot Camera Crystal - take in-game screenshots. Can print out via NPC shops later on.