- Ask for a certificate. Make sure it's from an impartial diamond grading authority (e.g. Gemological Institute of America, Jewellers Association of Australia)  or an independent appraiser who is affiliated with a professional organization (like the American Society of Appraisers). This is especially important if you're buying a stone you haven't seen, such as from the Internet.
- Look through it. Diamonds have a high "refractive index" (meaning they sharply bend the light that passes through them). Glass and quartz have a lower refractive index, meaning they sparkle less because they bend light less, even when they've been cut nicely (because the refractive index is an inherent physical property  which is not altered in any way by a nice cut - unless, technically speaking, the cut induced a permanent strain on the crystalline lattice).
- If the diamond is not mounted, turn it upside down and place it on a piece of newspaper. If you can read the print through the stone or even see distorted black smudges, then it probably isn't a diamond. 
- If the stone shows any sign of double refraction, it may be Moissanite (silicon carbide), a gemstone that is so similar to a diamond that even jewelers can have a hard time telling them apart. 
- If the diamond is mounted, you should not be able to see the bottom of a diamond looking directly from the top. 
- Observe the reflections. A real diamond's reflections usually manifest in various shades of gray. If you see rainbow reflections, you're either dealing with a low-quality diamond or a fake. 
- Take the side view. Most imitation diamonds are crafted to sparkle at the top, but check to see how it sparkles and reflects from a side angle. A real diamond will be just as reflective all around, whereas a fake is more likely to be duller when seen from the side. 
- Fog it up. Put the stone close to your mouth and breathe on it. If the stone stays "foggy" for 2-4 seconds, then it is definitely not real. Real diamonds will have cleared by the time you look at them. Be warned though - some jewelers cap cubic zirconia bases with real diamond which will, of course, clear. 
- Drop the stone in water. Many fakes have a real diamond cap but a false base, and under water, it is easier to see a joint if there is one.
- Weigh the stone. cubic zirconia weighs approximately 55% more than diamonds for the same shape and size.  Use a carat or gram scale to compare the stone in question to a real diamond.
- Check the setting and mount. A real diamond is not likely to be set in a cheap metal.  Stamps inside the setting indicating real gold or platinum (10K, 14K, 18K, 585, 750, 900, 950, PT, Plat) are a good sign, while a "C.Z." stamp will give away that the center stone is not a real diamond. 
- Put the stone under a UV light. Many (but not all) diamonds will exhibit blue fluorescence under an ultra violet or black light, so the presence of a medium to strong blue confirms that it is real. The absence of blue, however, does not mean it is fake; it could simply be a better quality diamond.  If you see a very slight green, yellow, or gray fluorescence under ultraviolet light, it may be Moissanite. 
- Rub the diamond on corundum sandpaper. If you see scratches or it is nicked, then it is not a real diamond (diamonds are harder than sapphire, which is the same stuff as corundum). Keep in mind, however, that some types of sandpaper have diamond in them, and that can ruin a diamond.
- Test it with a heat probe. Real stones disperse heat quickly and they won't heat up with the probe. This takes about 30 seconds and is often done free of charge. It also doesn't hurt the stone the way some other ways of testing will.
- Have the diamond x-rayed, real diamonds do not show up on an x-ray, glass, cubic zirconium and crystals all have slightly radiopaque quailities, diamonds are radiolucent.
- If you take the stone for an independent appraisal, expect to pay between $35 and $75 in the US, and make sure the stone never leaves your sight. 
- Diamonds do scratch glass, but so do many imitation stones.
- Diamonds,yes even good ones, can and do cast rainbow refraction. Please have a gemologist confirm this.
- Some diamonds are lab-created or synthetic, but they are still "real". They cost a fraction of what a mined diamond costs, but they are (for the most part) chemically the same as "natural" diamonds. Telling the difference between a natural and a synthetic diamond is beyond the scope of this article and is best determined by a professional.
- There is no way to be 100% sure that a diamond is real unless there is a certificate. If you buy a pawned item, something off a table at a market, or an item off of a website, you are taking a risk.
- DO NOT hit your diamond with a hammer! A popular myth states that, because it is so hard, you can hit a real diamond with a hammer and it will not break. This is FALSE. A diamond, when hit with a hammer, will shatter just as easily as a crystal or other stone, albeit much more expensively.