The sapphires formed in the magma, but apparently not from the magma! If the sapphires formed in the magma, it is likely to be from a syenite, which is a silica-poor magma formed from low degrees of melting: as this magma sits in the crust, the corundum would in time solidify (precipitate) as the magma cooled. In this location in the Eifel, the sapphires came not from the deep mantle, but formed in the crust in locations where the magma heated the local crust.