How does agate form?

Agates are formed in gas cavities in volcanic rock, as well as in cracks and holes in metamorphic and sedimentary rocks. Solutions high in silica content filled these cavities to form silica gel which hardened to the agate we know.

Generally, the formation temperatures are now thought to have been between 40 and 270degrees centigrade, perhaps more to the lower end of this range. Theory of agate formation has evolved with better microscopes and continued study over the years. Books by Roger K. Pabian are recommended.

At any rate, there were no human witnesses to the formation of agates, and we are still learning about these fascinating gems.