Madagascar green opal on brass

This stone comes from the province of Betroka very south of Madagascar. It is a green opal. On a solid antic brass base, it is luminous. A mineral showing several shades of green. Like all opals, it is a silicon dioxide hydrate. And even by shaking it, you will not hear the 6% of water it contains, like all Opals. Its color would be due to Nontronite, a clay mineral rich in iron (and that we have just discovered on Mars!) This green opal is therefore a silicification of Nontronite. Hence the transparency over the green color. It is better to avoid immersing it in water, because if the silification is not homogeneous, the water could reach Nontronite, and melt it. And as there was water on Mars a long time ago, we did not find these beautiful green opals on the red planet!