Hi guys, I have been reading all your messages about mica powders on nails and may be able to help you with some of your problems.
There are several grades of mica powder on sale - some for general crafts and some for cosmetic use. The cosmetic grade are pure mica powders with no added binders or chemicals and they are fully skin safe so they have no asbestos or heavy metals in them which many craft micas do.
The craft micas on sale for rubber stamping and other decorative effects are mixed with various resins and binding agents which will not fully disolve in the nail varnish or gel bases and can feel gritty. More importantly they will stop a full bond on the nail which can lead to chipping or flaking as well as the polish not setting firmly. They are also not considered skin safe
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It has to be said that they are incredibly beautiful though and as you can mix the colours together you have un unlimited palette to create with
I hope this helps a little.