Any properly cut and polished transparent gemstone should appear brilliant across its entire breadth when the observer looks into the stone with the light source directly above its table. Under these conditions an improperly cut stone will show a "dead" spot in the centre, and a needle or similar object placed near the culet will be visible due to leakage of light through the pavilion. In other words, it will act as a window pane, not as a mirror. The needle will not be seen if the stone is properly cut.
Referring to the accompanying table of refractive indices, it will be seen that diamond has the highest refractive power of any natural gemstone, sapphire much less, and topaz and quartz still less. Particularly in colorless stones can one gradually begin to separate, by careful observation with the unaided eye, stones of high and low refractive power. In colored stones, such separation can also be made between different species, if the stones are of the same hue and of about the same depth, or tone, of that hue. On the other hand, a light topaz-colored quartz might seem more brilliant than a much darker tourmaline. The reason for this is that more Light is absorbed by the brown stone, and it therefore reflects less light to the eye. But for the purpose of making a general statement regarding comparative brilliancy, one can select a gem of anyone species in the table as being more brilliant than one of any species lower in the table, assuming that both are correctly proportioned and well polished.
Synthetic rutile, which is sold under a variety of trademarked names such as "Miridis", "Kenya Stone", "Titengem", "Titania" and "Tirugem" is advertised as being much more brilliant than diamond. Other factors being equal, this would be true, for the refractive indices of synthetic rutile are 2.61 and 2.90, compared with 2.42 for diamond. However, a comparison of a well-cut diamond with a well cut synthetic rutile will show that the other factors are NOT equal and that the diamond will appear more brilliant. Synthetic rutile is less transparent than diamond. It absorbs a significant portion of the spectrum, so it cannot return to the eye of the observer the high proportion of white light that is returned by diamond. Furthermore, it possesses a phenomenal power of dispersion. This means that much of the light totally internally reflected does not return to the eye as WHITE light but as spectral colors, or "fire." In addition, it is difficult, if not impossible, to polish surfaces on synthetic rutile that produce a luster even approaching that to be expected from a material of such high R.I. .Diamond takes a higher and finer polish. The higher degree of transparency and better polish of diamond more than make up the difference between the refractive indices of the two stones. Therefore, diamond is actually more brilliant than synthetic rutile.
Comparative Importance Of Factors Governing Brilliancy
From the above discussion it will be seen that the brilliancy of a stone depends on several factors. One of these is REFRACTIVE INDEX, since it affects the amount of light REFLECTED at the first surface (i.e., from the crown facets) and determines the size of the CRITICAL ANGLE, which, in turn, determines the amount of light that is totally reflected from the pavilion facets as well as that lost through leakage.
A second important factor is the correctness of the PROPORTIONS. A large amount of light can be lost through the back facets of a poorly proportioned gem, even one of high R.I.
