I have a medal of Mary Blessed Mother and it has ct 18750 on the back of it .I think it' s very old gold but would anyone have any ideas to prove it's gold

18750-caratage system of gold measurement for instance the 18 means simply 18kt gold as most Americans are use to seeing however 750 is 750 parts of 1000 which means it is 75% pure gold. Most people are use to seeing 24kt as pure gold, 18kt is 75% pure 14kt is 53.324% pure{might be slightly off on the exact decimal on that one its been awhile since I've had to use it}It is most defiantly gold by the stamp but you would need to take to a dealer to tell for sure.Or the best and FREE way to find out is take to any "We Buy Gold" place at most malls or any pawn shop. They will test it for free .However do not sell them your piece they generally only pay 25% to 40% of what the gold market price is for the day .You can find that out by looking up gold prices for the day on any search engine . Keep in mind 1 ounce of gold is a troy ounce there are 24 pennyweights in a troy ounce or around 31 grams so to figure the price you would get you need the weight of the piece and again any pawn shop will tell you for free. Multiply the weight by 75% of the gold price for that day.Ex: necklace weighs 3 pennyweights and gold is selling at $1000. An ounce today 1000 divided by 24 is $41.67,41.67x3 is $125.01 so that would be just the amount the metal is worth . The style, design,craftmanship,age,and other factors add value. If they use grams instead of pennywieghts just substitute 31.10 .I hope that helps