A diamond does not glow. It has no light source, no fluorescing coating and no power supply. Every photon leaving the top of a well cut stone entered it moments earlier from the room.
What makes it look otherwise is a piece of applied optics worked out largely by trial and error over five centuries and then solved mathematically in 1919. It is one of the few places where a luxury object is genuinely an engineering artefact, and the physics is more interesting than the marketing.
The starting conditions
Diamond is carbon arranged in a cubic lattice where every atom is covalently bonded to four neighbours in a tetrahedron. That structure produces the properties everything else follows from.
Refractive index of about 2.42. Very high. Water is 1.33, ordinary glass around 1.5. This is the single most important number in the whole story.
Dispersion of 0.044. Dispersion is how much the refractive index varies with wavelength, and it determines how strongly white light splits into spectral colours. In the trade this is called fire.
Hardness of 10 on the Mohs scale, the top of it, which is why a diamond holds a polished facet edge for centuries while softer stones round off.
Thermal conductivity higher than any other bulk natural material, roughly five times copper. This has nothing to do with appearance and everything to do with how gemmologists test stones, which we will come back to.
Total internal reflection is the whole trick
Here is the mechanism.
When light travelling inside a dense material hits the boundary with a less dense one, it either refracts out or reflects back, depending on the angle. Below a certain angle from the perpendicular it escapes. Beyond that angle it reflects entirely, with no loss. That threshold is the critical angle, and it is determined by the refractive index.
For diamond, the critical angle is about 24.4 degrees.
That is an extraordinarily small window, and it is what makes diamond behave the way it does. Light entering the top of the stone strikes the angled facets of the pavilion, the cone-shaped lower half, at angles well beyond 24.4 degrees, so it reflects internally rather than passing out the bottom. It bounces, usually twice, and exits through the top where you are looking.
A cutter’s entire job is to arrange facets so that light entering the crown is delivered back to the eye rather than leaked out the base.
Why proportions decide everything
This is why cut matters more than any other property, and why it is the one buyers hear least about.
If the pavilion is cut too shallow, light striking it arrives at less than the critical angle and passes straight out the bottom. The stone looks washed out, with a dull disc in the centre. Cutters call this a fish eye.
If the pavilion is too deep, light reflects once, arrives at the opposite facet at a bad angle and exits through the side. The centre goes dark, which is called a nail head.
Between those failures is a fairly narrow band of proportions that returns most of the light through the top. A stone cut correctly and a stone cut badly can have identical carat weight, colour and clarity and look nothing alike.
In 1919 a young engineer named Marcel Tolkowsky published a mathematical analysis of light paths through a round diamond and derived the proportions that maximise return. The modern round brilliant descends directly from that work. The industry has spent a century refining the numbers rather than replacing them.
Fifty-seven facets and what each does
The standard round brilliant has 57 facets, or 58 if the point at the bottom is polished into a small flat culet.
They divide into functional groups.
The table, the large flat facet on top, is the main entry and exit window.
Crown facets, the angled surfaces around the table, refract entering light and, critically, split it. Dispersion happens mostly here, because light crossing a boundary at an angle separates by wavelength.
Pavilion facets below the girdle do the reflecting. These are the ones that must exceed the critical angle.
The girdle is the widest band around the middle, where the stone is held by the setting.
The three visual effects the trade names separately are all consequences of this arrangement. Brilliance is white light returned to the eye. Fire is dispersed spectral colour. Scintillation is the flashing produced as either the stone or the observer moves, which is a function of how many facets there are and how they are arranged.
There is a trade-off between brilliance and fire, and cutters choose where to sit on it. More white return tends to mean less colour separation. Old cuts from before electrification favoured fire because they were designed for candlelight. Modern cuts favour brilliance because they are designed for electric light and daylight.
Why antique stones look different
Worth knowing if you ever handle one, because the difference is immediately visible and it is not a defect.
Old mine and old European cuts were made by hand and judged by eye. They have small tables, tall crowns, deep pavilions and an open culet that reads as a dark window when you look straight down. Under low warm flickering light they produce broad slow flashes of intense colour.
Put the same stone under an LED downlight and it looks sleepy compared with a modern brilliant. Put a modern brilliant under candlelight and it looks comparatively flat.
They are different optical instruments designed for different illumination. Neither is a better stone.
Where colour comes from
Pure diamond is colourless. Colour comes from defects, and the type of defect determines the colour.
Nitrogen is the commonest impurity and produces yellow tones. Diamonds containing measurable nitrogen are classified Type I, which is the vast majority.
Boron produces blue and, unusually for a gemstone, makes the diamond an electrical semiconductor. These are Type IIb and they are very rare.
Radiation exposure in the ground produces green.
Pink is the strange one. Pink diamonds do not get their colour from an impurity at all. They get it from plastic deformation of the crystal lattice, physical distortion of the structure under enormous pressure, which alters how the lattice absorbs light. The colour is structural rather than chemical, which is part of why pink diamonds are so rare and why the colour cannot be reliably reproduced.
This connects to a genuinely Australian piece of geology. The Argyle deposit in Western Australia supplied more than 90 per cent of the world’s pink diamonds and was unusual in sitting in a lamproite pipe rather than the kimberlite that hosts most diamond deposits. The conditions that produced the lattice distortion there have not been found in comparable quantity anywhere else. The mine closed permanently in November 2020 and Rio Tinto’s final inventory tender concluded in October 2025, which means 2026 is the first year where every Argyle pink in circulation is a stone that already exists.
How gemmologists actually tell stones apart
This is where thermal conductivity becomes useful.
The classic diamond tester is a thermal probe. It touches the stone, pushes a small amount of heat in and measures how fast it dissipates. Diamond conducts heat away far faster than almost anything else, so the reading is distinctive. Cubic zirconia and glass fail instantly.
Moissanite defeats a thermal tester, because silicon carbide also conducts heat well, which is why testers now often combine thermal and electrical conductivity measurement. Moissanite can also be identified optically: it is birefringent, meaning it splits light into two rays, so looking through the crown at the pavilion facets shows visible doubling of the facet edges. Its dispersion is roughly 0.104, more than twice diamond’s, which is why it throws far more rainbow colour and reads as slightly theatrical to a trained eye.
Laboratory-grown diamonds are the hard case, because they are not simulants. They are diamond, with the same lattice, the same refractive index, the same hardness and the same thermal conductivity. No handheld tester distinguishes them.
Identification relies on growth artefacts. High pressure high temperature stones often show metallic flux inclusions and distinctive strain patterns. Chemical vapour deposition stones show striated growth structure. Many lab-grown diamonds phosphoresce after ultraviolet exposure in ways natural stones usually do not. All of this requires spectroscopy and magnification in a laboratory.
Which is why the paperwork matters. In late 2025 the Gemological Institute of America stopped applying its D to Z colour scale and flawless to included clarity scale to laboratory-grown diamonds, assigning them instead one of two descriptive categories, premium or standard. The grading language itself now separates the two products.
The economics the physics produced
Because laboratory-grown stones are physically identical and supply is limited only by reactor capacity, prices behaved exactly as you would expect. Average prices fell 20 to 30 per cent against 2024 alone, and some categories are down more than 90 per cent from their peak. Above one carat, lab-grown now commonly sells at 5 to 10 per cent of a natural equivalent.
There is something quietly remarkable in that. A material whose value rested almost entirely on geological scarcity turned out to be straightforwardly manufacturable, and the market split into two products with identical physics and utterly different price curves.
What did not happen is a collapse in natural fancy coloured stones, because those depend on formation conditions that are genuinely difficult to reproduce. The physics that makes a pink diamond pink is lattice damage under specific pressure over geological time, and that has proven much harder to fake convincingly than simple crystal growth.
Fluorescence the property nobody agrees about
Roughly a third of diamonds fluoresce under ultraviolet light, usually blue, because of nitrogen defects arranged in particular configurations.
The trade has argued about this for decades. Strong blue fluorescence can make a slightly yellowish stone appear whiter in daylight, since daylight contains ultraviolet and the emitted blue counteracts the yellow. In that specific case it is a free optical upgrade.
Very strong fluorescence can also make a stone look faintly hazy or oily, though this affects a small minority of fluorescent stones.
The pricing is where it gets odd. Fluorescent stones generally trade at a discount, even where the effect is neutral or beneficial, which means a buyer who understands the property can often get a better stone for less money. It is one of the few places in this market where informed buyers are genuinely rewarded.
Why hardness is not toughness
A distinction that costs people stones.
Hardness measures resistance to scratching, and diamond tops the Mohs scale. Toughness measures resistance to fracturing, and diamond is merely good rather than exceptional.
The reason is cleavage. That neat cubic lattice has planes along which the bonding is weaker, and a sharp impact aligned with one of those planes can split a diamond cleanly. Cutters exploit this deliberately to divide rough crystals.
The practical consequence is that a diamond will outlast almost anything in a drawer of jewellery without picking up a scratch, and can still chip if it catches a hard edge at the wrong angle. This is why low profile and bezel settings matter for anyone using their hands, and why the girdle, the exposed band around the middle, is where chips almost always happen.
If you want to see the effect for yourself
Two experiments that need no equipment.
Hold a stone table-down on a piece of newspaper. A well cut diamond will not let you read the text through it, because light entering the pavilion is being reflected rather than transmitted. A poorly cut stone or a simulant often will.
Then look at the same stone under three lights: a window, an LED downlight and a candle. The proportion of white brilliance to coloured fire will shift visibly. That shift is dispersion and critical angle doing their work in real time, and it explains why a stone that looked extraordinary in a showroom can look ordinary at home under different lighting.
Any decent jeweller will let you do both. Workshops that cut and set their own work, Stelios Jewellers among the Australian ones, tend to enjoy this sort of question rather than deflecting it, because the people at the bench are the ones who actually think about facet angles for a living.
The summary
A diamond is a light trap. Very high refractive index gives it a critical angle of about 24.4 degrees, which means almost everything entering the top gets reflected internally instead of escaping, and a cutter’s job is to aim that reflection back at your eye.
Everything else, the grading scales, the pricing, the marketing, sits on top of that one piece of geometry.
This article is general information only. Figures are approximate and vary with specific material. Confirm specifications and certification with a qualified gemmologist or jeweller.
