Per this wowhead page, there are a large number of missing gems in dual color variants.

In addition to this, there is a quest that gives Jewelcrafters a chance to "Perfect" cut a green quality gem via Gem Perfection. I am unsure how to reflect this in the code since it is not created via a spell.

Perfect gem example: https://www.wowhead.com/wotlk/item=41482/perfect-accurate-huge-citrine