Maybe someone will come along and correct me, but I've never heard of a .22 mag cylinder for a jet. Jet barrels are .222 while .22 mag barrels are .223, which is why .22 long rifle, .22 mag convertible guns generally don't shoot as well as a dedicated .22 lr revolver.
Are you sure this isn't a .22 long rifle cylinder?
These are fun guns, but they are finicky. Before you shoot .22 Jets you need to thoroughly degrease the chambers and the cartridges to prevent them from setting back on firing and locking the gun up.
The plum coloring is not especially uncommon, I believe it is due to the different compositions of the steel and the way it interacts with the blueing.
Value depends on several variables, 3 or 4 screw, whether you have the box and tools, but probably the value would be in the $700 to $1000 range depending on exactly what it is.
Oh, and I would shoot it! I've got a mint 8-3/8 4 screw that is a blast, literally. Just be sure that you wear good hearing protectors, these thing are loud!