I just realized something about my last two posts.
What if gems scale by one rate and jewelry by another? Specifically, gem value is multiplied by five every six steps and jewelry by ten every six steps? I’ve adapted the exponential values table so a “times ten per +6” column (labeled ‘Metal’ below).
That is, if you start with a gem worth 100 gp and get +6 (size and quality) to its value, it’s worth 500 gp — as much as the next grade of stone. However, if you craft a piece of jewelry and get +6 (purity and quality) to its value, it’s worth ten times as much — that silver ring is worth as much as a regular ring made of gold.
The math vs. arithmetic problem again, I think, though it does make for a smoother transition between the metal values and it does mean you need less metal for it to be relevant when gems are present. Even the most exquisite gold ring (+6 = *10 value) is worth only about 20gp, hardly noticeable against the 1,000 gp sapphire (though the two together are 1,600 gp).
Keeping this for posterity, in case I have the thought again.