When crafting a lamp, if you have fire crystals of different illumination levels, they are not counted together. Instead, it seems that the stack that appears first in your inventory is the one the game checks first.

Example 1: the two unlit crystals are counted, so I can make the lamp. Example 1

In example 2, the only change is that I dropped the unlit crystals and picked them back up, so now the lit crystal is counted first (and only), so I cannot craft the lamp. Example 2