Hyalite Opal (Opal-An), Erongo Mountains, Namibia
Hyalite opal is almost always bright green and can be found all over the world. The opal from the Erongo Mountains in Namibia, however, is among the brightest. Here's a nice four inch piece shown in SW and natural. Note that the exposure is set for the individual rock; if exposed similarly to other specimens, this guy would appear as a bright green blob.