Question

My problem is that some of the gold jewelry I have is not marked, but it is definitely gold. I was told that I would be able to extract the gold if I left the jewelry soak in HNO₃ (strong pure nitric acid for several days). After several days, all that would be left is the gold. Is it actually that easy or is there more to it?

Answer

Put simply it is not that easy.  There are a number of alloys of gold, each containing a different percentage of gold along with alloying metals.  These metals can themselves be soluble or insoluble in nitric acid.  If you take a piece of gold and put it into nitric acid the nitric acid can only react with the surface of that piece of gold (which is usually only a small percentage of the total material there).  The nitric acid may dissolve some of the alloying metals there, but the inside of the metal will remain unchanged unless it is such a small percentage of gold that the material will fall apart as the alloying metal is reacted away.

The usual way to attack this situation is to dissolve the gold in question in its entirety in aqua regia or another system then recover your gold and other materials that may be present one at a time so you don't get cross contamination between the constituents (silver can muck this up, precipitating as the chloride).  Iron sulfate, sulfur dioxide, and other reducing agents can be used to drop the gold out of solution after it's all dissolved  Again though, if you don't know the constituents of what you are dissolving then those things can contaminate your final material. Recovery and Refining of the Precious Metals⁽¹⁾ gives a lengthy procedure for isolating gold from 'secondary sources' that would be very useful in your situation.

Because your jewelry is not marked you could attempt to dissolve a small amount in some aqua regia then do some qualitative tests on it to see exactly what there is in the solution.  Assuming you found out that your gold alloy was a gold/copper alloy the procedure used in Ammen's book for purification is to dissolve your alloy in molten copper to reduce the kt, pour into water in order to get smaller particle size. Dissolve the alloy in nitric acid, then dissolve the residue in boiling aqua regia.  The solution is boiled down then diluted with the gold being precipitated with sulfur dioxide.  Again, this is just the method outlined in that text but my personal experience with gold refining is fairly small aside from assorted attempts to dissolve in bromine/hypobromite solutions.

(1)Ammen, C.W. Recovery and Refining of Precious Metals. 2nd ed. Alberta, Canada: Deep Rock Resources Inc. 1993