When you send in gold to a refiner you do have to fill out a lot of paper work, most of it for anti money laundering, patriot act etc stuff and be a business.
And, when the refiner is done, they send you the amount its valued at minus there fees. They do not do your taxes, that is your responsibility. Just as if you were paid cash for a job, your responsible to do what you think is right.
If you are setting up as a business and finding enough gold to make it worth while and will need to sell it. All your expenses are tax deductible against what you find/make. This means your mileage, your tools, equipment, Gas, pans diving suite, dredge, permit fees, claim fees etc are all deductible.
In fact if you set it up as a business and dont find anything and dont make any money you can still wright it off against your taxes. Talk too an accountant is my advice based on what you make, and if its a good idea or not.
Otherwise, most smaller who sell a pinch of gold here and there dont worry about it. It would be like selling off your old junk at a garage sale...do you pay taxes on that and call it income? most dont.
Some states do have a tax on commodities removed from the land. If I recall in Wyoming the old law on the books I believe requires a dollar an ounce or maybe it was $1.50 on all gold recovered.
I only know of one prospector that ever did that, he gave them a check at the end of the summer for anywhere from $12 to $20 for the gold he found, but most dont bother as the amounts are so small for the weekend prospector for the gram or two they find they are not going to bother paying the state the 5 to 15 cents for what they found.
Most gold found is kept as a specimen or a keepsake. Its an unknown value as a commodity as its not refined. As it sits as specimen gold it is not income.
Do you pay taxes on that old baseball card in the attic that may be worth $500? One really can not asses a value unless its sold. And until then, you cant establish a value or income. If you pay taxes on that $500 baseball card and then find you can only sell it for $150...well. you see where this is going.