Just a general comment here that, while platinum coins may be around in various D&D worlds, they really shouldn't be found in low-magic settings. Not just because of the extreme rarity of the metal, but because its melting point is over 2000 degrees Fahrenheit. A kingdom that issues platinum coins either has access to specialized spells to melt the platinum, pays a dragon to breathe on it, or something similar.
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...Or, they are dwarves.
related to this, it is noted that some pantheons' realms lack or don't use some metals. Egyptians don't use iron, for example. There are definitely some groups that use platinum on the planes, but as you said, it is still extremely rare and thus, even more valuable than gold.
Also,
Mithril = Titanium.
T/F ?