For measuring the brightness of some materials, some dated industrial methods cite a Magnesium Carbonate block standard to be set to Y = 100% in reflectance. This is a very old top-of-scale standard and HunterLab does not recommend using it.
Instead users should update their methods to the current industry standard of a PRD Perfect Reflecting Diffuser as 100% top-of-scale for reflectance. A Magnesium Carbonate plaque will read slightly lower than a PRD standard, typically around 97%.
If you are determined to use a Magnesium Carbonate top-of-scale, a source for Magnesium Carbonate powder that can be pressed into a plaque can be found at:
Magnesium Carbonate, Pentahydrate, white powder
(Magnesite, Carbonic Acid Magnesium Salt)