What about synthetic emeralds are those a possibility at all? I lack both the knowledge and inclination to look into them. could they be used for the front of the ring??
Chatham Emerald is the common synthetic Emerald but it would be a poor choice compared to Sapphire. Emerald, synthetic or natural, is extremely brittle. Chatham Emerald, even though it is synthetic, is also expensive.
The only other green stone that would be somewhat suitable would be green Spinel or Tsavorite/Demantoid (green Garnet). Spinel is harder than Garnet but is softer than Corundum (Sapphire.) I really like Tsavorite but finding material of the right size and color is unlikely as well as extremely expensive. Demantoid can be really spectacular but is also extremely expensive. The only other green stone that immediately comes to mind is Chrome Tourmaline, which is too easily damaged.
I would venture to guess it would take well over 30 carats of material to cut a piece big enough to make the green portion for the top of the ring. While it would be possible it certainly isn't probable because it would be obscenely expensive (read as many thousands of dollars) and difficult to do- if you could even find a gem cutter to do it.
Making a custom mold and producing the piece in green ceramic is really the only viable solution (in my mind) if you must have a translucent green material that is extremely durable- and that really only makes sense if you are going to produce a lot of identical rings. The total production cost is still going to be high.
So that leaves you with plastics as the only really somewhat affordable option.
If anyone can think of anything else I'm all ears.