The discovery in 1952 of some river pebbles of jadeite-jade on the North side of the Motagua River Valley (MRV) in Guatemala strongly suggested that at least part of the long-lost geological source of Mesoamerican jade was local. Another river pebble has been discovered, but this new one provides two archaeologically and geologically significant differences: (1) it is the first ever to be found on the South side of the MRV; (2) it presents a new different petrological kind of jade. The confirmation of the mineral species jadeite was achieved with this rough piece of rock without any kind of sample preparation using only a Raman Microscope (RM) which is now a reliable polyvalent non-destructive technique for archaeometry.