When the item is used, it will produce 10-25 platinum coins according to your faith skill level *.
The item says it can be used every 72 hours. While it CAN technically be used that often, it will only produce the platinum coins once a month. The coin rewards reset on the first of every month no matter how late in the previous month you used it, HOWEVER, the 72 hour cooldown will still apply. This means that if you used it at noon on the 30th of one month, you won't be able to collect the rewards until noon on the 3rd of the next month.
Using it displays the text "I give pocket money to you! You must spend it usefully!" If it is ready to use but you have already used it to produce platinum coins during the current month, it displays You'll have to wait until next month.
The displayed cooldown of 72 hours is not a bug; rather, under the Japanese settings, each additional use of the gem every month displays a new message from Yacatect. These additional messages are untranslated in the English version of Elona+.
The gem is located at 657 (Row 19, Column 30) by the character sprite zero-based numbering system. It is a god jewel, which seems to be tinted slightly purple.