The desire for a token to be worn (with a great deal of pride) while a loved one was at the front became known as a sweetheart brooch. The tradition continued after the war in the form of battle town brooches, souvenirs bought in the various areas around the front.
They are still highly collectable and there is a wide variety of prices, although the ones bearing squadron insignia tend to attract the best prices. It’s easy to believe there is gold and silver in them but usually they are only brass and silver plate – even if they are finished to high quality. Look for relevant hallmarks on any purporting to be of precious metals and one other tip is that they’ll usually have a safety catch rather than a pin to ensure they don’t come off accidentally.