use descrimination on detector, test with a silver dime, see if it sounds good, i have the silver test kit too, nice little tool, hard to tell some times though even with those two, have also had stuff marked 925 that wasnt silver.
Re the ?silver : try placing the earring in clear ammonia; if it is silver, the clear ammonia will turn light blue ( chemical reaction of silver with copper content, so I'm told).
Please let us know if it turns out to be silver..it's a nice looking earring.
If beach hunting is something you're going to be doing more of then you may as well invest in a couple of loops, a test kit, a diamond tester, and a basic electrolysis setup. A lot of the items you'll find are going be pretty rough, especially the older stuff, and some of the markings will be very tiny and often squeezed in the most unlikely spots. But, a lot of time the markings can mean a great deal of difference in the item's value, rarity, etc., not to mention that these marks can also often tell you a great deal about the item, its maker, etc.