I never had a problem with the dust mechanics - although I did have a brief problem with my understanding of the dust mechanics. Where did you see a problem?
For dust to reconstitute into an object, it appears the gem beam must terminate on the same space where the dust is. The ray can either terminate because it's blocked and cannot go further (by a wall) which will terminate the beam on the space before the wall or by hitting a destroyable object which will terminate the beam *on* the space of the object.