What does 585 on my nib mean?

If your fountain pen nib has the numbers 585 engraved it means that is a 14 carat gold nib (14c or 14k).

Gold purity is specified as a fraction of pure gold - 24 carat (24c or 24k) - so 585 parts per 1000 is the same as saying 14 parts per 24. In other words, 585 divided by 1000 and multiplied by 24 (585/1000 * 24 = 14).

Similarly, 750 on a nib means 18k gold (750/1000 * 24 = 18).