As well as increasing value of Fire and Earth, I think all gems should be transmutable for coin. Maybe for less than those 2 gems, but if it is made strategically viable, then it would kind of suck that only a few nations have starting gem income to make it possible in the early game where some would argue it was most useful. Taking EA Abysia as an example, 6F gems per turn, if it were bumped to 30g apiece, that's an extra 180g/turn if they decide to alchemize at the start to get a faster start. 4 turns of that is basically an extra fort, while someone who has no access to F gems has no viable option whatsoever in that department.
Actually, I'd probably say making F worth 30, S and E worth 25, and all others worth 20..... Would probably be a fair enough balance. Or you could say F/S/E 30, and all else 25, I don't care.
Though have to be careful, if you make it worthy for everyone, the 50% bonus could end up too powerful.