Maseki （魔石） literally means demon stone.
The stone can be used for fixing magical tools and restoring one's magical power. this stone is considered to be very valuable and can fetch up a hefty price in the black market when sold. Maseki is shown to come in a variety of color and this includes the blue colored pendant belonging to Julia.
Julia's pendant Edit
The famous pendant that Julia gives Conrad for protection when he was sent out on the front lines during the war, which is the same pendant that Conrad gives Yuuri for protection before his battle with Wolfram at the beginning of the series, is a blue maseki.