Governor Hickenlooper of Colorado says
“his office has been in contact with a professor from Stanford, who is experimenting with water that works as an atomic signature for marijuana, allowing law enforcement to tell if it’s legal or not.
“The goal, he said, is to make it possible that ‘if you pick up marijuana, you can tell if someone paid taxes or if it was contraband.’”
Any consumable marker to would have to be nontoxic, hard to counterfeit, inexpensive to apply, and easy to detect in the field.
This reminds me of the way we tax virtually the same substance if used in vehicles (diesel fuel) and exempt it if used for heating buildings (heating oil). The exempt stuff is marked with a red dye. Can marking work for marijuana concentrates? And how about green plant material?