How to do it
Take a page of newsprint, or draw a small dot on white paper. Put the loose stone flat side down directly on the text, so the pointed end faces up at you. Look straight down through it in good light.
If the stone is a diamond you will see no letters. Not fuzzy ones, not faint ones. The text simply does not come through. If you can read the print, or see a distorted version of it, you are looking at something with far lower refraction, which means glass, quartz or cubic zirconia.
Why the effect is so extreme
Refractive index measures how hard a material bends light. Diamond sits at 2.42, which is enormous, and a brilliant cut is engineered to take advantage of it. Light entering the top gets thrown back and forth between the pavilion facets and sent back up at your eye instead of continuing down to the paper.
| Material | Refractive index | Can you read through it? |
|---|---|---|
| Moissanite | 2.65 | No |
| Diamond | 2.42 | No |
| Cubic zirconia | 2.15 | Barely, distorted |
| White sapphire | 1.77 | Yes, blurred |
| Quartz | 1.55 | Yes |
| Glass | 1.5 to 1.7 | Yes |
Cubic zirconia is the interesting case. At 2.15 it is high enough that you will not read the text cleanly, but you will usually catch light or shape coming through in a way a diamond will not allow. If you are unsure, that ambiguity is itself a signal worth following up.
The limitation nobody mentions
It only works on loose, unmounted stones. Every setting ever made holds the stone table up and pavilion down inside metal, which is exactly the wrong orientation and blocks the light path anyway.
If your stone is set, the fog test is your best option, followed by a look at the facet edges under a loupe.
Not sure what you are looking at?
Photograph the stone and the app gives you a visual read on the 4Cs, an estimated value range, and what to check next. Free on iOS and Android.
Frequently asked questions
What exactly should I see through a real diamond?
Why can I not do this on my ring?
Does it work on moissanite?
What about the dot test?
Worth being clear: everything here, and the app itself, is an educational reference. At-home tests narrow things down, they do not certify a stone. Before you buy, sell or insure anything of real value, get it looked at by a certified gemologist or an independent appraiser.