Do some research reading where gemstones are close to you. For instance wee jasper, NSW for sapphire, take a couple of sieves and a pan. The sieves 1/2" , the other 1/4" to 'classify' or sort the material you dig from the ground / creek.
Things like gold and gems are usually heavier than most other rocks around so if your digging in water, you want to dig deep and look at the sands, pebbles and rocks you bring up.
Gemstones and minerals stand out completely separate from all other rocks you will be digging and extremely noticeable. If you live in a good area, you may be able to find something close and easy to find like large quartz crystals or sapphire / garnet which glisten with color and shine in the gravels quite noticably.
If its listed in history or on a map, its still there today. Some are very hard to find which requires a lot of exploration, or some may be at your feet when you park your car at a location.
If you can say what area you are near, I can post what you can probably find.
I'll also edit in that you need to purchase a miners right and abide to the rules of digging for the least environmental impact and keeping areas clean / tidy