I have heard that arsenic can be camouflaged in this way.
And to think that some Victorian ladies spiked their tea with it to create a pearlescence in their complexion ... truly a death pallor. Ah, the saga of Arsenic and Old Lace. WARNING! Anyone remodeling old homes with plastered walls should be wary of arsenic poisoning and wear suitable HAZMAT protection. An indicator of its potential is a horsehair binder in the plaster.