I bought a diamond watch off ebay. Bit risky you may be thinking. I took it out of the envelope (yes really!) and although it was fairly dark in the house the sparkling it gave off was absolutely stunning. Had it valued and it was very real. Diamonds just have a whole extra dimension which cubic zirconia etc do not have - nice as a lot of them are.
Cz are heavier than diamond which is why cz sellers say equivalent weight. What looks like one carat of diamond will weigh more if it is cz. Cubic zirconia will scratch glass.
The setting is often a give away. If there is light on both sides of the setting it is more likely to be a valuable stone than when it is sat inside a mounting.
Old rings with old cut diamonds have less sparkle so isn't always true that there isn't enough glitter. Unless you have a diamond tester or are an expert you need someone to check for you. Even with modern stones moisenite will sometimes give a diamond reading with a tester so unless you know what you are doing its better to get a profession valuation
yes, very easy, first thing after the look described above is to look for little gas bubbles and dirt marks - diamonds have these unless they are flawless, the worse the diamond quality, the more........the jewellers eye will show them up.
Carat, Clarity, Colour and Cut, the 4Cs of diamonds.
though, ultimately, brindal is right; the scratch test is also a useful gauge as glass and paste will be cut very easily, zircon as well but tougher, diamond is the hardest natural crystal.
yes glass test is worth o do...
But i think testing it at home or by own is difficult task. on should check out for authorizes gemologist. Only they people can guide you. or ask for certificate.