Warning, this is going to take a while!
Edit: Following the U-10 update gem charms will no longer be required to prevent gems from cracking. U-10 will also introduce some more options for obtaining bound brilliance and empowered and standard gems.
First, the basics. Stellar gems drop in U-9 adventure worlds from shadow gem boxes (the air, fire, and water worlds will drop their respective gem box type in addition to standard boxes), to enter a U-9 world you must be at least 10k power rank. Empowered gems drop from empowered gem boxes, which can be obtained through class power rank leaderboards, club adventurine merchant, and shadowy market (by using up lunar souls). Each gem has three stats; for this achievement you must upgrade all stats on nine stellar gems to 100% and upgrade the gem to level 25. To upgrade a gem stat use must use any of the three builder's focuses on a gem at the gem forge. To craft builder's focuses use the adventure's crafting bench, the materials are located under the 'gems' tab. To level up gems you need gem dust (red for fire gems, blue for water, and yellow for air), flux, and charms (jade clover, glittering horseshoe, lapis luckbug, or ninth life). Charms are used to prevent the gem from cracking, which would cost you a re-gemerator (purchased for 500 cubits at the store).
How fast you progress towards this achievement depends largely on how much bound brilliance (material required to craft any of the three focuses) and charms you can get. Since you can only earn a certain amount of bound brilliance a week here are some tips to get max bound brilliance a week asap.
Get all classes to at least 5K power rank. You will earn up to 20 bound brilliance, and 12 empowered gem boxes a week from the class power rank leaderboards from this.
Craft the legendary tome 'Book of Bound Brilliance' at the adventure's crafting bench. This will net you 5 bound brilliance a week.
Make sure to log on during the holiday event periods and do the special event quests, you will normally be rewarded bound brilliance from this.
Pay attention to the leaderboards to see which ones you need to do to earn gem charms and empowered gem boxes.
Purchase the 'Jade Clover Journal' from the store. It is a legendary tome that will net you 100 jade clovers a week. Unfortunately it costs credits. So spend that doe or grind some flux to purchase credit pouches from the marketplace.
Grind out gem dust on Gem Day (Wednesday). You will also be able to earn up to six lustrous gem boxes on Gem Day, which may drop any of the three focuses.
Only equip standard gems of at least 550 initial power rank, and empowered gems of at least 800 power rank. This will start the gems out with a higher stat % and cost you less focuses in the long run.
I can not confirm if all gems have to be maxed on the same character. If anyone has additional advice or corrections please comment or message me.