Much of the so-called "pixie dust" on the market is not genuine, but is a plant-based substitute with similar properties.  True pixie dust, a natural material shed from the pixie's skin, has little magical value in its native state, but holds enchantments placed upon it for later use, releasing the magic when blown at or dusted on a suitable target.  For that reason, it is best not to track pixies using magic.   The invisible layer of pixie dust that surrounds them absorbs most spells, and the pixie can then turn them back on the caster.  Few experiences are as humiliating for a pixie hunter as being frozen by a paralysis spell aimed at a pixie, then being taunted (and frequently robbed) by the intended victim.