Reason why Ruby is Red and Sapphire Yellow?
Its all Al2O3 !
Crystalline form of Al2O3 (aluminum oxide) is called Corundum. It is found in many colors and shades, every shade may be named differently in different languages, in English deep red corundum (aluminum oxide) crystal is known as ruby and every other shades are known as sapphire, but both terms have single name in Arabic that is Yakut.
Why so many colors?
If every crystal is made up of Al2O3 then where did these different shades of color come from? The answer is some other elements like iron, chromium, titanium, vanadium, copper and magnesium replace aluminum atoms in few crystals of the stone. Even though these are the atoms which shoot up value of the stone, these elements in the crystal are called impurities by chemists!
Important Facts About Ruby and Sapphire
Al2O3 with less than 0.01 percent of titanium in the crystal makes colorless sapphire
Chromium atoms give the sapphire pink hue, more number of chromium atoms increase redness of the crystal and it tends towards the formation of ruby , crystal with at least one percent of chromium is deep red ruby. Chromium atoms absorb yellow-green light from white resulting in red color stone. But don't assume that further addition of chromium atoms will increase redness, you can make out by seeing the graph (2) below what will happen if every atom of aluminum is replaced by chromium in the crystal.
Trace amounts of vanadium in the crystal create purple shades in the sapphire.
Most beautiful and expensive sapphire of blue hue and tones indicate the presence of iron and Titanium ions, Fe2+ and Ti4+,, but in only 0.01% quantity.
If only iron atoms are present then stone is seen very pale yellow or green in color.
shades of yellow sapphire (yakut e safra)
what's the relation with emarald?
Above information is all related to ruby and sapphire which are known as Yakut in Arabic on the other hand a precious stone known as emerald in English and Zamarrud in Arabic is green in color. The crystal of emerald is Beryl: beryllium aluminum silicate (Be2Al2Si6Ol8) and the green color is formed with the impurity of chromium. Now the amazing thing is that chromium as impurity in colorless Corundum (Al2O3) makes red ruby and the same element in colorless beryl (Be2Al2Si6Ol8) makes out green emerald.
the reason of this phenomena is a little bit related to deeper knowledge of electrons' energy level and ligand field explained below in the graphs, or simply you can say it nature's creativity.