Almost 30 million years ago, an asteroid or comet impact fused Saharan quartz grains into desert glass.
There have been some interesting news stories recently about ancient water here and on Mars. They are hard to describe in plain English, though–at least
Diamonds aren’t the only mantle-associated gems. Most of the upper mantle is made out of peridotite, a combination of olivine and pyroxene. It is named after the very famous gemstone it contains–peridot.
A hundred years ago, no one had a clue that disaster can strike us suddenly from the sky. Today we have the best knowledge and finest equipment humanity has ever had to meet the challenge, if it arises.