Trying to tax marijuana plant material on the basis of its potency (say, THC content) may be impractical because different labs routinely report wide variances in measured potency from a single sample.
But there’s a processed marijuana product that may be fungible enough for a potency tax: “concentrated cannabis oils, known in various states as ‘butter’, ‘shatter’, ‘wax’, and ‘BHO’.” That product looks fungible to me – video here.
And there’s reason to think this form of marijuana may be particularly troublesome. Here’s a report from http://radicalruss.com/high-times-medical-cannabis-cup-in-san-burn-a-doobie-2013/:
Dabbing, which to the uninitiated is the use of concentrated cannabis oils, known in various states as “butter”, “shatter”, “wax”, and “BHO”, has all but consumed the culture of the younger, hedonistic pot aficionados.. . .
[Russ Belville] counted at least four times when EMTs were trucking out someone on a stretcher to an awaiting ambulance, someone who had taken far too big of a dab and passed out. This is such a common occurrence at these events now that I’ve taken to keeping count. That doesn’t count the young man I found violently puking, shaking and weak in a state I remember from my last trip to Los Angeles, when I, too, had too much of a dab and ended up face down puking in a gutter. Now, this is a spectacle you’d expect at beer-served events, but when our drug comes with blow-torches and immediate passing out, the image will be a hard one to get past the mainstream as just harmless old weed.