Ruby on Marble (#45)

Lustrous translucent 3/8" ruby prism on white marble.
Asia Jegdalek, Sarobi, Afghanistan 1 ¾" x 1 ½" x 1"


Learn More

Ruby is one of the four precious gems (see Learn More for #19 to learn about gems), but there is no mineral named ruby. Ruby is a variety of corundum, as is sapphire, and the difference between them is simply color. By definition, ruby is red, although the shade of red can vary. Sapphire is any other color of gem corundum. Carat for carat, ruby is more valuable than diamond.

So, what's a carat? A carat is a unit of mass equal to 200 milligrams. It is used for measuring gemstones and pearls. Of course, it isn't just size that determines the value of a gem. Clarity, color, cut, and other factors have their impact.

Another question: If the same mineral (corundum) can have different colors, what causes them? The answer, as you might suspect if you have read some of the other "Learn More"s that deal with color, is an impurity - some element not found in the chemical formula but present in very small amounts. In the case of ruby, that element is chromium, Cr. Corundum is Al203, and chromium oxide is Cr203, and the two have virtually identical atomic arrangements, so even though Cr is a somewhat larger ion than Al, small amounts of Cr can enter the corundum structure. The Cr3• ion has 3 d electrons, and surrounded by 6 oxygen atoms, the five d orbitals are of different energies. So when white light strikes ruby, the photons of the right energy promote electrons to higher energy d orbitals, and the energies (wavelengths) of those photons are removed from the spectrum. What you see is what is left of the spectrum. Color caused this way is said to be due to crystal field effects. The color-causing elements (chromophores) in sapphire, as opposed to ruby, are different and may work by different mechanisms.