RM RICOMAX Metal Detector for rent, $10/day. In addition to generic metal detecting, also has special zinc, nickel, and copper modes. $50 deposit. VENMO, PayPal, and cash accepted.

It's best if you can narrow the search space. Searching for, e.g., a ring in a field with this detector can be daunting because of the number of false positives from bottlecaps, soda tabs, keychains, coins, etc.

To maximize success, recreate the lost item scenario with a similar item, and practice detecting this similar object, while setting the SENS (sensitivity) and DISC (discrimination) dials.

SENS dial: All the way right will be more sensitive. This gives the detector a bigger detection range, but will result in more false positives.

DISC dial: All the way to the left should detect all metals. It also has copper/nickle/zinc/etc. specific modes at different dial positions as you turn it to the right.

For gold, you probably want to set it all the way left (though this will maximize false positives). If it's an alloy and you have a duplicate ring or item, you might have luck finding a sweet spot where it beeps for this item and not for other common metals. This will be tremendously helpful if it's a large search area.

If you've lost something especially small and valuable, you might also consider theringfinders.com or reaching out to a metal detecting club (many expert finders have a "pay-what-you-think-is-fair if it is found" policy or a nominal travel fee). Metal detecting can be quite challenging depending on the item and where it was lost.