I respectfully disagree with the answer you were given
The gain you mention in your question is a capital gain. It is a long term capital gain since in your example you held teh commodity over one year.
It will therefore be taxed at one of the three long term capital gains brackets the IRS has established for 2018. Which one of teh three you fall in, 0 15 or 20 % will depend on your total income that year, namely on your tax bracket.
If you are in teh 10 or 15 % tax brackets, then your long term capital gain tax is zero.
If you are on teh highest tax bracket for that year, tehn your long term capital gain tax will be 20 %.
If you are on any of the other tax brackets, then your long term capiatal gain tax will be 15 %
I hope this helps