I don't really like this gem. It's Prase opal, a variety of common opal but with nickel giving it the neon bluish-green hue. This and Chrysoprase are similar in appearance but distinguished by their varying Refractive Index and Specific Gravity and Prase opal tends to be more translucent.
This particular opal came from recently discovered veins on Mount Iyobo in central Tanzania where Chrysoprase and Prase opal have been mined for more than 20 years. The new discovery has some of the most intense bluish green material mined in this area.
They both form in discontinuous veins within metamorphosed ultramafic (iron rich) rocks with the dominant host rock being altered serpentine. The top gem weighs in at 11 carats but I don't like the green hue. What I covet is the specimen in the rough, I love this piece. I love the matrix stone above and below that gives a richer green to the Prase opal.
The picture also brought back a dream I once had, a very long time ago but one of those dreams that tend to stick in the mind. I was in a room entirely made of Chrysoprase but not smooth, the walls and floor were like this piece of matrix. I have no idea what it meant and it didn't give me any liking for the gem. My mind is full of little mysteries like that. It's probably full of holes like Swiss Cheese as well.
I'd put in a map to Mount Iyobo but I ditched goggle earth for taking up too much computer room.