The name for this item is "Purachiaru Kinoko". Platinum is "Purachina", capitalizing on the chi also representing "ti" - Platina, another name for Platinum. (Like was done with Giratina). Platina is the first result in an online Japanese translator, but only because it shares the first three letters - the weapon goes on to have the "iaru" which cannot fit, and does not have the "na". Since the shield and stave names match each other:
Happy/Pleasure, Premium/Limited, this should be synonymous with "Precious", and should be an adjective. Since it's clear "Platinum" was gotten by picking the first thing that came through an online translator, I think something like "Palatial", pronounced Puh-LA-chi-al would be much closer to the intended meaning.
pre⋅cious
/ˈprɛʃəs/ Show Spelled Pronunciation [presh-uhs] Show IPA
–adjective
1. of high price or great value; very valuable or costly: precious metals.
pa⋅la⋅tial
/pəˈleɪʃəl/ Show Spelled Pronunciation [puh-ley-shuhl] Show IPA
–adjective
1. of, pertaining to, or resembling a palace: a palatial home.
2. befitting or suitable for a palace; stately; magnificent: a palatial tapestry.
A Precious mushroom being a costly, extravagant mushroom, and a palatial mushroom being one fit to be in a palace.
