Diablo II: Resurrected

There are five tiers of gems: Chipped, Flawed, Normal (no preceding word), Flawless, and Perfect. Combine three of the same type/tier gems in the Horadric Cube (achieved early in the quests of Act II) to get the next tier.

Moving up through the tiers goes as follows:
3 chipped amethysts -> 1 flawed amethyst
3 flawed amethysts -> 1 amethyst
3 amethysts -> 1 flawless amethyst
3 flawless amethysts -> 1 perfect amethyst

The only action needed for this achievement is that last step: creating the perfect gem. This can be a perfect diamond, topaz, sapphire, ruby, emerald, amethyst, or skull. If you're exploring thoroughly, picking up every gem you come across, and keeping them all stashed without using any, you'll probably be able to do this around the end of your first normal playthrough, maybe a bit into your nightmare playthrough. If you're playing multiple characters that can share a stash, bringing all of their gems together will help you get there.

Keep in mind that you'll need a perfect gem of each of the six types (excluding skull) to create a Prismatic Amulet to complete "All the colors of the rainbow".