Many processes were invented to create artificial gemstones. Some processes are extremely complex and expensive, others are very common.
Very rare and interesting specimens are available on https://www.gemgazer.com/man-made-gems-66-c.asp
GIA publishedan interesting introduction to synthetic stones https://www.gia.edu/gem-synthetic
Natural Zincite is a very rare gem. This one is a byproduct of an industrial process, as it is not totally natural, not totally artificial. It was formed inside the chimney of a zinc treatment facility in Bresnau Pologne. Some are lab-grown (totally artificial) but microscope seems to show ashed trapped inside the crystal, which is the signe of it’s provenance.
Synthetic rubies made with the Verneuil flame fusion method. They are extremely fluorescent under uv.
Beautiful artificial color change sapphire! The pleochroism is amazing with beautiful red flashes, ans it chnges from blue to honey and vivid purple, between a sapphire and an alexandrite, cubic zirconia