If you subscribe to the long held definition
of a gemstone being rare, then synthetics cannot
even be called gemstones. We call them "synthetic versions".
Synthetics are not new. A. V. Verneuil produced
ruby using a a flame fusion method in the late 1800's. And
that method is
in use today, although more sophisticated methods produce synthetics
aka "lab" or "created".
Synthetics of a given type have all the same mineral properties of
natural gems. That include crystal structure, trace elements that cause
color plus the same hardness and durability of their natural counterparts.
But synthetics are lab created and, depending on the type,
are produced in mass quantities. They may look pretty,
but they are not natural so they are not rare so they
are not gems, in our opinion.
Synthetics can be extremely beautiful. They are usually
cut to near ideal proportion since the material is so
cheap. They are generally flawless to the naked eye.
There is nothing wrong with buying or wearing synthetic
ruby, sapphire, spinel, etc .. just as there is nothing
wrong with hanging a copy of a Rafael in your living room.
But they are not Rafaels nor are they gemstones.
Just be aware that if you have a flame fusion 5 carat
synthetic ruby ring, most knowledgeable folks will spot
it as a synthetic right away.
Simulants
A material that is used as a substitute for a gem material,
but is of a different chemical composition is a "simulant".
The best example of a simulant might be the substitution
of CZ for diamond.