Gold/Silver Purity

A coin's precious metal purity can only be edited and displayed up to the nearest thousandth, but some coins have purities that extend beyond the thousandths. An example of this is the 100 Pounds - Elizabeth II (4th portrait; 1 oz Fine Gold) from the United Kingdom (https://en.numista.com/catalogue/pieces14379.html), which has a purity of 91.67%/.9167 gold, but it is displayed as 91.7%/.917 gold due to these limitations.

The difference amounts to only 10mg of gold. Unless you're a pharmacist, isn't 91.7% close enough?

50ct are 50ct

Hello,
This is not a limitation. You can enter a fineness up to 6 digits (99.9999%). I just made the request to modify the coin you mentioned as example. You can see here an example of coin with a fineness of 91.67%: https://en.numista.com/catalogue/pieces140151.html