These green gems are called olivine crystals, a common mineral found in Hawaii's lava. When the green gems are turned into jewelry, they are called peridot. But, when the volcano reupts, it blasts apart molten lava allowing the green olivine minerals to be separated from the rest of the melt and actually fall from the sky as tiny gemstones.
Several places in Hawaii have green beaches because of a high concentration of olivine. Actually, olivine is one of the most common minerals below the surface of Earth; it is just so difficult to find it separated from the parent rock and even harder to find it of gem quality.
It may be common underground, but olivine falling from the sky...is not. Is this planet remarkable or what?!