I could go on for an hour on how to identify diamonds but I will give you the short course.
First of all experience is my number one tool. During a normal day I could look at 100+ different stones and I spent 2 years at gemology school learning to do so. It amazes customers that I can look into a loupe and tell them what they have. I'm sure they tried that several times and didn't see the the flashing neon sign in the stone to tell them what they have. They don't believe me. They want me to put it in some kind of star trek machine with flashing lights and computer voice that tells them what it is. That machine doesn't exist. Yes there are electronic diamond testers but they falable. They are just another tool to eliminate possibilities not a final authority.
Generally the only tool I use is a 10x loupe and a good one not one of those $10 specials. A good loupe is color corrected and has no distortion. Expect to pay $40+. I use a microscope on tough ones.
Let me give you a quick rundown on what I look for;
Keep in mind that our goal is to eliminate everything but a diamond not to identify everything along the way. Like I tell my counter guys it is trash or treasure. There are only three stones to concern ourselves with Diamond, CZ and Moissanite. It's a process of elimination of possibilities.
1) Karat of gold. The higher the karat of gold 14-18 or if platinum the higher possibility it is a diamond. In 25 years I have never seen a diamond in silver. 10k is a 50-50 chance that the small stones are diamonds and that a bigger center stone is not a diamond, sapphire or whatever.
1a) Prongs. If you have high karat gold or platinum. Six prongs holding a round center stones is a winner. The outside edge or girdle of a diamond is the weakest part of a diamond. Yes they can chip and crack if you bang it hard against something. The more gold you have surrounding and protecting the center stone generally means something.
2) If it looks too good it may not be a diamond. CZ's have more fire than a diamond and look better than a diamond.
3) Diamond is a natural occurring crystaline mineral and generally has imperfections (carbon specs or white wispy inclusions) in the stone just like looking into a semi clear quartz crystal. A perfect diamond is too rare and expensive to put into a ring. 99% of all diamonds put into jewelry have inclusions/imperfections in them. Use your loupe. If you see inclusions it is a diamond. No inclusions, a really really really nice diamond or in 99.9% of the time it is a CZ.
4) CZ's are amorphous like glass, they do not form a crystal. NO INCLUSIONS but can be cracked. They feel waxy, diamonds do not. The CZ girdle or edge if not polished will look waxy like a bar of ivory soap. CZ can chip or the edges can abraid. They chip and wear like glass. Often small shell shaped chips can be seen on the girdle and facet edges. Sometimes big chips or cracks. The facet edges (the angles where the facets meet) will appear rounded or worn like a river rock often not symetrical or properly aligned at intersects. Look too white. Culet not polished (see below).
5) Diamond girdle looks clear crystaline. Good diamonds will have a faceted girdle. All facet edges are sharp and symetrical. Most have inclusions. A natural is often visible on the girdle. A natural is a small section of the actual crystaline skin that was not polished. Diamonds often have bearding, cz's do not. Bearding is caused when the original crystal is put on a lathe to take off the corners creating a round stone. If you look at the girdle from the top down and see little white wisps starting from the edge inward, that is bearding. Off colored yellow or brown indicate generally a diamond. Polished culet. (see below for def.)
6) Moissanite can be tough for the beginner. Looks like a diamond, feels like a diamond almost as hard as a diamond. Polished girdle like good diamonds. However there are some differences. The conclusive test is it has some electrical properties. Which means you need a special machine to test, not cheap. But with some practice your old loupe can be put to work. Moissantite has an off color generally a light seafoam green tinge or light brown. It is also doubly refractive. Doubly refractive means that light when entering the stone splits into two paths. This causes a doubling or mirroring effect when looking into the stone. It is very faint and can generally be seen looking straight down into the stone at the culet. The culet is the point where all the lower facets meet at the bottom of the stone. If you look carefully you will see two culets because of the doubling effect. If you have or know someone with a Peridot look at it through your loupe and you will know what I am talking about. They have a very strong doubling. You think you are going blind. Still a good Moissanite can be worth $400 a carat compared to a CZ.
Important you can't see any of the above reliably if the ring or stone is dirty. Clean, clean clean!
If you have something that nice that you can't make a proper identification take it to an expert. Remove the stone from the mounting. A final determination may need to be made by a specific gravity test. If the expert needs to take it out of the mounting spend the money because you more than likely have a home run!
Comparing is important, look at known diamonds, look at your unknowns, hold then next to one another. You will learn.
Happy Hunting!!
BustMeOut Bob
Steve