I have read many customs declarations forms. (And possibly more carefully than you have.)
The most recent form I read was the Australian form, where the relevant question is:
“Are you bringing into Australia AUD$10,000 or more in Australian or foreign currency equivalent?”. Nothing about valuable goods as an alternative to cash. (You have to make a similar declaration if you are taking currency out of Australia.)
There’s a separate question asking about goods obtained overseas with a combined value of more than AUD$900. But that’s an entirely different matter (as the very different monetary threshold indicates). That question is concerned with the evasion of tax on imported goods, and you only have to answer the question with respect to goods that you bought overseas (or duty-free). And of course you don’t have to answer it at all when you’re taking goods out of Australia. By contrast, you have to answer the cash question no matter where you got the cash (and of course there’s no import duty on cash, even if you have obtained it overseas), and whether you’re bringing it in or bringing it out. If you’re wearing or carrying AUD$10,000 of jewelry you don’t have to say anything about that in answer to the cash question. But if you are bringing it in, and have obtained it overseas, they you need to declare it in answer to the imported goods question.
The US form asks about “cash or monetary instruments over $10,000 US or foreign equivalent”. Nothing about jewelry or other goods there. Again, you have to declare it both entering and leaving. A separate question asks travellers to declare the value of goods they are carrying that they purchased or obtained abroad; unlike cash, this question only applies on entry, there’s no $10,000 limit, and the question is confined to what you have obtained abroad.
The EU customs declaration form is similar - entering or leaving, you have to declare currency or bearer-negotiable monetary instruments that you are carrying, regardless of source, over a threshold of EUR 10,000, but there is no general requirement to declare goods that you are carrying of a similar value. Goods only have to be declared on entering, and only if you obtained them abroad and are importing them; in that case the threshold for declaration is EUR 430.
So, in all the countries that I know of, you can lawfully avoid carrying (and therefore declaring) cash by buying jewelry, and then wearing or carrying it across the border. But do this at home, before you leave; if you buy it while you’re abroad, you’ll need to declare it as an import (and in that case you’ll almost certainly end up paying duty on it, which you wouldn’t do with the cash).