As I understand your question, you are not asking for an expensive course which might send you gems to evaluate or a summary of how to do gem ID, but instead just want to practice identifying natural vs synthetic inclusions. I think this is a great way to learn, as there are a number of these (and other) separations that need to be made via loupe or microscope inspection. The first thing I would do is to get a good zoom gem stereo microscope. You will find new models from China for sale and older B & L and American Optical models for sale also. About $350 would be my estimate of what one will cost. You’ll likely find a 7-30X model and for some separations you will need up to 60X, so you may need to purchase a 2X doubler or a set of 20X eyepieces to add to the 10X standard set. If darkfield lighting is not a feature of your microscope, you can use washers of some kind along with a suitable blackout sheet to create darkfield lighting. You’ll also need some LED lights on goosenecks for direct lighting from various angles above and below the gems. If the microscope you get doesn’t have a gem holder, you can get one on ebay and install it on the scope. Also get some “blue tack” to mount and move things around with, if needed. If you haven’t a microscope, use a loupe, but there are some separations that require the microscope…also, altho’ gem appraisers do have to use a pocket loupe alone to evaluate on occasion, looking at stones to evaluate cut and condition is something most easily done with a microscope.
I would pester my friends for jewelry to look at, but I think you can find small, included gems for sale at Gem Rock Auctions and there are filters to narrow your search down to small, individual stones…you may find other sites where you can buy such, too. I saw some small sapphires for sale for $30 or so each. You could also look for cheap facet rough. If you don’t see it listed, you could query the dealers, as they often buy parcels and cull out the junk and might be happy to sell you some.
While there are some sources of good photos on line, if you want to see all the inclusions, the three volume set by Koivula and Gubelin is the go-to source for original picutures. These volumes are very expensive, but you could probably get them (one at a time) through interlibrary loan for free and look thru them. Some of the earlier editions or other books by K & G might be had cheaper…Lidicoat’s old Handbook of Gem ID also has some photos in it and might be available cheap, IDK.
The synthetic vs natural differentiation is the real tough nut to crack in gem ID and I congratulate you for wanting to tackle it head on. And, while there are other ID helps (such as UV response), the use of the microscope is the royal road to conquering this particular conundrum.