Stone · Carbon, cubic, and cut by a person
Diamond
Four grades, of which one was made by a human and decides everything, and three that the market prices in the wrong order.
Cut, in three numbers
A round brilliant returns light when its proportions send rays entering the crown back out through the crown rather than leaking through the pavilion. Table percentage between 54 and 58, total depth between 59 and 62.5, crown angle near 34.5 degrees and pavilion angle near 40.8. A stone cut outside those ranges is usually cut deep, which holds weight where nobody can see it and pays for it in dead light.
The plot diagram, which nobody reads
A grading report includes a map of the inclusions. It is the most useful page and the least examined. An SI1 with a small crystal near the girdle, hidden by a prong, will look cleaner to the eye than a VS2 with a feather directly beneath the table, and the price difference runs the other way. Reading the plot is the largest single arbitrage available to a private buyer.
Fluorescence, which is priced irrationally
Some diamonds glow blue under ultraviolet light. Strong fluorescence is discounted in the market, sometimes heavily, on the assumption that it causes a hazy appearance. In practice that applies to a small minority of strongly fluorescent stones, and in a lower colour grade a faint or medium blue fluorescence can make a stone look whiter. It is one of the few places where the market inefficiency favours the informed buyer.
Grown stones
Laboratory grown diamond is the same material with the same properties, produced by high pressure high temperature or chemical vapour deposition. It is identified by instruments rather than by eye and is inscribed on the girdle. Prices are a fraction of mined equivalents and continue to fall, which makes it excellent value as an object and close to worthless as a store of value.
What the money buys, in order
Fix cut, set colour by the metal, buy clarity by eye, then buy weight just under a round number.
How to check it
- Read the plot diagram before the letter grade. Position beats classification.
- Ask to see the stone in several lights: a window, a restaurant level of light, and a spotlight. Only the middle one resembles real life.
- Check the measurements on the report against the weight. A one carat stone measuring under 6.3 mm is cut deep.
- Look for the girdle inscription and match it to the report number. This is what ties the paper to the stone.
Care
- Clean in warm water with washing up liquid and a soft toothbrush, from behind the stone where the dirt sits.
- Ultrasonic cleaning is safe for diamond in sound settings and unsafe for many other stones.
- Have prongs inspected every twelve to eighteen months.
- Take rings off before the gym, gardening and anything involving a grip. Most damage is to the setting, not the stone.