Carefully wash off any soil or mud that is hiding the details of the design and construction so you can start to identify it. The page below has an interesting article about the different types of button construction, which will help you to describe it:
Don’t use harsh cleaning products – a tiny amount of soap and plenty of clean water with a very soft brush or cloth are all you need to start with. Try to stop cleaning when the design elements are clean enough to be identified – going further than that may damage the metal. If you want to clean it whilst away from home, spit won’t hurt and I’ve used puddles in the past!
If you think your button may be from a British naval uniform, this website by the Royal Museums Greenwich is very interesting:
https://www.rmg.co.uk/collections/objects/search/Button
This site has a great many excellent photos of British naval buttons:
https://www.colchestertreasurehunting.co.uk/navy%20buttons.htm
This pdf, showing the fronts and backs of naval buttons, was clearly prepared in connection with an auction so it may have vanished before you read this: if that’s the case, please let me know and I’ll remove the link:
https://www.jagermedals.com/_pdf_files/militaria_pdfs/j3627_large.pdf
Although apparently dedicated to Malta, this site has some very clear information on British naval buttons:
https://www.melitomnes.org/05-3-2-2-wwii-rn-officer-buttons-fl-large.php
If you find a website that helps you to identify a button, please send me the link and I’ll include it here to help other people.
If you can’t find what you are looking for please contact me.