For those who prefer a text guide, here's what you need to know.
To unlock this achievement, you need to obtain five crystals. There are at least 15 crystals in the game and most are hard to miss, so this is easy to accomplish. Crystals can be found by either unlocking certain doors (typically done by splitting up your lumini) or by passing through some walls.
It's unlikely that you won't find at least five in a playthrough, but if you don't, the crystal count is cumulative across playthroughs.