Where do treasure symbols come from?
In article #1 on the topic, the astute researcher will have noticed that a healthy chunk of commonly-encountered symbols in various publications seem to all trace back to J. Frank Dobie’s book, Coronado’s Children.
Likely, and according to a lot of writers, general themes were common, dictated by secret or obscure commandments even, but often as not, each person or persons caching a treasure would put their own special spin on the conventional meanings in order to maximize security. An exception might be highly-organized groups, required by dictate to make it easy for any of the anointed in their organization to recover the treasure years, or even generations, into the future.
Nonetheless, totally nonsensical markings would not be of use to anybody. Consider that ancient symbology is a great well from which to draw inspiration (modern architecture is a perfect example of hidden symbology). This legend on an 1850 map of New California (David Rumsey site) provides historical insight as to the symbols associated with important minerals and precious metals by explorers of the New World.
It seems that the language associated with this legend is Italian, but the names for Gold (oro), Silver (argento), Platinum (Platina), Mercury (mercurio), Copper (rame), Iron (ferro), Lead (piombo), Coal (bitume), and so on seem clear enough. It is interesting to see that Silver is associated with the moon, the symbol for Gold (oro) matches well with other references, and a decent guess might be that the minerals are ranked in importance. After all, Gold and Silver require Mercury for extraction from many sources, so these three come before Copper, for example. Also, physical similarity seems impotant, as the symbols for Platinum and Mercury both have little moons incorporated into them, as they both resemble the main prize: Silver.