No it does not work like that. The value is dictated when the commodity or investment is sold for FIAT and then you pay income tax on that.
From the (mind-numbing) government of Canada tax site:
"Deemed proceeds of disposition
This is an expression used when a person is considered to have received an amount for the disposition of property, even though the person did not actually receive that amount."
... and ...
"Eligible amount of the gift
Under proposed changes, this is generally the amount by which the fair market value of the gifted property exceeds the amount of the advantage, if any, received for the gift.
Under proposed changes, the advantage is generally the total value of all property, services, compensation, or other benefits to which you are entitled as partial consideration for, or in gratitude for, the gift. The advantage may be contingent or receivable in the future, and given either to you or a person not dealing at arm's length with you.
Under proposed changes, the advantage also includes any limited-recourse debt in respect of the gift at the time it was made. For example, there may be a limited-recourse debt if the property was acquired though a tax shelter that is a gifting arrangement. In this case, the eligible amount of the gift will be reported in box 13 of Form T5003, Statement of Tax Shelter Information. For more information on gifting arrangements and tax shelters, see Guide T4068, Guide for the Partnership Information Return (T5013 forms)"
Sorry for the emetic. My point was just to illustrate that the State is very aware of how to turn commodities into fiat equivalents whether or not the parties to the transaction actually used fiat, and indeed whether or not a transaction even took place!
http://www.cra-arc.gc.ca/tx/ndvdls/lf-vnts/dth/glssry-eng.html#proceedsOf course, rules vary from one jurisdiction to another, but when one State entity finds a way to make a tax grab then others tend to follow suit.