His team found a layer of igneous rock in Southwest China, and used the marine fossils to date it to a huge volcanic eruption 260 million years ago. (discovermagazine.com)
Gems are usually found in igneous rocks (mainly pegmatite dikes) and in contact metamorphic zones. (daviddarling.info)
I still collect igneous rocks from sevral of these volcanic sites close to Sydney. (dotearth.blogs.nytimes.com)