You could cast your bullion into jewelry, but why would you? 24ct is too soft to be worn without the worry of it ripping apart and falling off.
My wife and I, (Calamity Jane), through our connections in Italy (the gold capital of the world) and due to the lucky fact that I had made so many contacts while living in Vicenza, (the gold capital of Italy), we got into the specialty jewelry market where we contracted to have X number of custom fraternal rings cast each Quarter ... it was fun and profitable until 9/11 happened and all the bases were shut down. It would also not be doable now because of the state of the economy and the price of gold ... but it sure was neat while it lasted. And we ended up with some casting grain leftover that we store in a safety deposit box at the bank.
We've traveled a lot so here is what I think I know about your questions.
Already covered the bit about casting 24ct gold into jewelry. About the lowest grade you want to cast into any kind of jewelry is 18ct.
About traveling through various customs with jewelry. There is a rule ... declare it or risk losing it somewhere along the way, especially upon your return home. And it'll be non-negotiable if it happens. What you get away with here you may not get away with at any of your stops along the way, and visa versa. And you do not want them wanding you and the buzzer going off around your wrists and neck and all of the sudden you're pulling off enough gold to weigh down the valuables pan ... because they will start looking on what you declared as yours before you departed (see Form 4457) and if it is not on that computerized list then either pay taxes on it or lose it.
Now, what I am basically concentrating on here is when you return to the U.S. so here is how it is best explained when it comes to your Form 6059B declaring what you bought while overseas ... and covering how to cover yourself before you leave with regard to your valuable items.
When you enter the US, you fill out a customs declaration.. Known as a Form 6059B (for the English version). When you sign it, your are making a legal declaration with penalties for failure to disclose things and the overall accuracy. Be careful that you declare EVERYTHING as Customs *does* have the right to assess penalties for a false and/or incomplete declaration.
Before you leave the US, if you KNOW you are going to travel overseas with items of a high value or items that can be reasonably have been purchased new while you were overseas, GET THEM REGISTERED with the CBP. You can register items for FREE, there is no charge to do it.. If you live close to a CBP office, take them to the CBP office in person and fill out a Form 4457, called "Certificate Of Registration For Personal Effects Taken Abroad"..
The CBP officer will verify the item and the description you listed. He/she will then stamp the form 4457 and return it to you to keep.. This stamped form acts as "proof" to the CBP that you had these items before you went overseas and as such, they are NOT subject to taxes when you come back to the US.
If you do not live close to a CBP office, you can also have this done at any staffed US Post Office. They will have the 4457 form and can do the certification on behalf of the CBP in those cases when a person does not live close to a region CBP office.
Alternatively, you can bring original receipts showing that you bought these items earlier and not on this current trip.
Remember that in the end, YOU have to prove your items origins. Customs (CBP) does NOT have to prove that you bought them overseas, YOU have to prove that you didn't.
I hope this helps answer your questions.