To have or take on the appearance, form, or sound of; imitate
a. Not natural or genuine; artificial or contrived: “counterfeit
rhetoric that flourishes when passions are synthetic” George F.
Will. b. Prepared or made artificially
So simulant gemstones are gems made to look like or imitate certain
natural gems, so CZ is a simulant.
Synthetic are not natural and are prepared artificially but are
chemically identical to their natural counterparts. So lab-made
corundum is synthetic.
I’ve only just my head around the difference myself!