A diamond is born - Diamantschleiferei Michael Bonke

c) Wrong proportions of the stoneOf the many superlative qualities that diamond possesses as a material, the high optical densityis arguably the most important one. Diamond is the material with the highest optical density andthus the highest refractive index (n = 2.42). A diamond owes its great significance as a gemstoneto its high refractive index. The sparkling of a gemstone largely depends on what percent of theincident light is reflected based on the phenomenon of total reflection, or how much light is lostthrough partial reflection and absorption on the reflective surface or by escaping from the stoneon to the back wall. This is where the reputation of a diamond as the king of all gemstonesbecomes evident: it is the only natural material (and only in the cut form of a brilliant), thatmanages to reflect 100% of the perpendicular incident light on the stone based on total reflection.No other natural gemstone can be cut to do this. This is because even in case of a zircon, theoptical density of which is the closest to that of a diamond’s but slightly lesser, there is no facetconstellation, in which the stone reflects a 100% of the perpendicular incident light.A ray of light is totally reflected without any loss of light, if it falls on the interface of an opticallythicker medium (diamond) to an optically thinner medium (air), if it is at an angle to the interface,which is smaller than the total reflection angle of the optical medium. This angle is 65 degreesand 34 minutes in case of diamonds. So, every ray of light that falls on the facet from the insideat an angle that is smaller than 65 degrees and 34 minutes istotally reflected. Compared to that: the total reflection angleof quartz is 49 degrees. The comparison between a diamond,zircon and quartz shows the problems of total reflection.Let us take a ray of light which falls perpendicular to the tableon an outer facet of the crown near the girdle:The light ray entering the diamondvia one of the crown-facets, is beingdiffracted, since it enters through thesurface at an angle. When hitting theback wall of the diamond the ray hasan angle to the pavilion facet which issmaller than 65 degrees and thereforeis reflected totally. It is being thrownonto the opposite pavilion facet and isreflected again totally, since again it hitsat an angle smaller than 65 degrees. Itthen hits the table-facet from inside thediamond at an angle of 90 degrees andleaves the stone.At hitting a zircon, the light ray isdiffracted less than with the diamond,because of the smaller optical densityof the zircon. Therefore after a doubletotal reflection it does not reach thetable-facet from inside the zircon, butarrives at a crown-facet. Here howeverthe lightray is hitting a third time at anangle below the total reflection angle,and is reflected a third time totally. Theray therefore cannot leave the stone atthe crown and instead, invisible for thespectator, at the pavilion.With the quartz the problem of the thirdtotal reflection does not arise at all. Afterthe first total reflection, the light rayarrives at the opposite pavilion facet atan angle bigger than 49 degrees. Hencethe ray is leaving already after the firstreflection through the pavilion and is lostfor the spectator’s eye.