Magic Crest (also known as Magic Seal in the SNES/PS version) is an accessory in Chrono Trigger and Chrono Cross. Effects vary between games.
Chrono Trigger
Increasing both Magic and Magic Defense by 5 points, this item can be worn by any character and is obtained in the Black Omen inside a chest.
Chrono Cross
Increasing Magic by 4 points, this item can be worn by any character. It is commonly dropped by FATE and is a rare steal of the same enemy. In Home World, the Magic Seal can be found in a chest in the sewers beneath Viper Manor. It can be made using 1 Mythril and 3 Scales and sells for 1125G.
Name Origin
Crests are components of heraldry. This crest represents a magical organization, idea, or ritual and so it boosts magical affinities. Official artwork for this accessory bears what appears to be a Pentagram, which is often an occult symbol. It may also be a Star of David.