Ah, you're saying things backward, you mean that you think sapphire is the 2nd most valuable, and ruby is 3rd. I don't agree with that, because there's too much space between their supposed values, given the wide variety of gems that we have. On the other hand, when Shallan was buying those books, the man responded with "Two emerald, three sapphire" which would be most natural if, like you said, it was the next most valuable denomination.
I also assume that soulcasting usefulness isn't the only thing determining relative worth of the gems, their respective rarities are probably a big part as well.
So far I'm not a big fan of the currency system in SA, look at US currency for example. we've got pennies, nickels, dimes, quarters, $1 bills, $5 bills, $10 bills, $20 bills, $50, and $100. So, that's 10 different denominations (no one uses $2 bills, or dollar coins), the difference between a penny and a hundred dollar bill is x10000. In SA we've got 27 or 30 different denominations, and the difference between emerald broams and diamond chips is only x1000. It seems to work fine, but it's pretty cluttered.
Looking at the quote saying that Shallan used all nine colors of sphere, I think she means actual color, I don't think she counts white in that, so that would be nine colors plus white, and so we're back to ten types. This really makes the most sense. I mean, why wouldn't they use all ten types of gem as currency? Why exclude one?
It might have something to do with Talenel.
I don't think the Heralds advertised after that last Desolation that they'd left one of their own to an endless cycle of torture. Besides the readers, I think only the other Heralds, Hoid, and maybe a few others knew about Taln.