Nimbus Silver Holographic Microflake

Is the nimbus silver holo made of mica or Polyethylene flake glitters

This product is made from Polyethylene.

Particle size please

This microflake is a 0.004" hex, or about 101 μm.

How much per gallon should be used?

Not more than 50 grams per gallon is ideal, but you can bump it up depending on the effect you're trying to achieve.