I know its been a while since my last theory, but school is crazy and I have run out of ideas. This is going to be about how gems could reproduce.
Since Gems can shapeshift almost anything they want inside and out, then they could also shapeshift their own genes and DNA. If a gem were to read a book on how genes translate into DNA then they could also read up on what physical appearances correspond to what genes, then they could translate those genes into a DNA sequence and, then once they have a partner they could turn it into the *cough* *cough* stuff that gets passed on, extract it and mix it with their partners. After that they have two option, a human baby or a gem baby. If they want a human baby then they would put their combined genes and inject it into a surrogate. If they wanted a gem baby, then they could take the same thing that kindergartens use to create new gems (look at other theory for more on that) and combine it with their combined genes and then inject it into the gemstone of their choosing.