A properly installed metal conduit can be the ground. If you install it right, you can install a grounded receptacle using the ground from the conduit.

The armor of old BX is not a sufficient ground. Neither is current type MC (though it will typically have a grounding conductor other than the armor). Type AC's armor (and it's integral bonding strip) can be used as a ground.

Type NM (ROMEX) with a ground wire is OK. Most of the smaller stuff (12, 14) has a full size ground (it's just bare). Larger stuff has a smaller grounding conductor because such is an acceptable size.

Art. 250.130(C) allows the extension of a ungrounded branch circuit to be connected to certain grounds to pick up the equipment ground.