J-type asteroids are asteroids with spectra similar to that of diogenite meteorites and so, presumably, to the deeper layers of the crust of 4 Vesta.
Their spectra are rather similar to that of the V-type asteroids but have a particularly strong 1 μm absorption band.
Examples are 2442 Corbett, 3869 Norton, 4005 Dyagilev, and 4215 Kamo.
References
J-type asteroid Wikipedia(Text) CC BY-SA