This is a new one to me...

[Ray]

An excellent grower of slippers near here uses Grodan Mini-Cubes and perlite as his phrag medium. My locally available perlite is simply too fine to use alone, so I started experimenting with a 3-phase approach, using LECA, Grodan, and perlite for a couple of phalaenopsis. The fine perlite tends to stick to the rock wool, providing more space between adjacent cubes. You can see that in the upper right of the attached image.

A few weeks ago, after repotting the plants into it, I was in a rush and neglected to get the perlite (coarse) dust off the leaves. Today I noticed permanent eruption bumps on the leaves where the material was. It's almost as if the cells grew up around-, and trapped them.

[WaterWitchin]

How odd. Any theories as to why?

[Ray]

Someone else said he noticed that when the perlite dust accidentally got in the crown of a paph.

All I can speculate is that the cuticle cells grew up around the dust particles.