I've recently began my quest to hit the level 10 upgrade item achievement.
So far I've been unlucky, think I've failed the level 9 upgrade like over ten times but that's a function of the low % chance of success. Every time I get enough Shooting Stars I take a chance in upgrading my level 9 item. Pity that by now, my progress along the game basically makes the level 9 item worthless so I'm just upgrading for the sake of the level 10 item achievement.
Unfortunately, that means I'm piling up way too many Amber and Amethyst gems and having too few Ruby's and Shooting Stars.
Maybe there could be a conversion option like in D2 where you get three lower level gems and can combine them into one higher level gems.
Of course the exchange rate should be tweaked substantially to reflect the general price environment. Something along the lines of 100 Ambers can convert into one Amethyst, 100 Amethysts into one Emerald and so on.