Why rubies and sapphires display such different colours?

Question

The gems (rubies and sapphires) are slightly impure forms of Al2O3(Aluminium Oxide), so why do they display different colours?

Answer 1

Hey Diana!

Nice name and Good question!

Gems like ruby (red), sapphire (blue), topaz (yellow), amethyst (violet) and emerald (green) are crystals of colourless, transparent Aluminium oxide (Al2O3) combined with a different, coloured metal oxide.

Let's see why ruby is red and sapphire is blue as your question is about the two of 'em only. Ruby is red due to the presence of Chromium while sapphire is blue due to presence of Titanium and Iron.

Let's see how the tiny-miny presence of these interesting elements changes the colour of these crystals so drastically, turning them from crystals of corundum (corundum is the name for rhombic lattice crystal of Al2O3) to gemstones!

When Al2O3 contains Cr3+ (Chromium ion) as an impurity, it appears red and is known as ruby. The Cr has unpaired d-orbitals. This Cr being similar to Al, replaces Aluminium in Al2O3 at places. Hence, instead of Al3+ and O2- framework, some places have Cr3+ with O2- framework.

When white light falls on it, the electrons in these half-filled orbitals get excited and absorb certain radiations and transmit the rest. The transmitted light is complementary to the absorbed and manifests itself as the colour of the gem i.e red. [In this case, violet and green parts of the white spectrum are absorbed and ruby appears red with a tinge of blue]

The absorbed light (photon) sends the unpaired d-electrons to a higher state of energy within the same d-subshell.

Same is the case with sapphires where Ti and Fe impurities' combination does the thing.

Difference is, here the process involved is of charge transfer. As light falls on sapphire, Iron ion transfers one electron to Titanium ion. This takes the crystal to a state of higher energy.

Fe2+ + Ti4+ --> Fe3+ + Ti3+

Light is absorbed and rest is reflected in this process. Here, all colours other than blue are absorbed.

One question arises- Excited states are not stable, why doesn't ruby or sapphire come back to its original state by releasing light of the absorbed colour?

Answer is- It doesn't happen that way. Excited state is done away with via another procedure. The energy is released in the form of heat and not light thus, raising the temperature around it by a bit.

Answer 2

Are Rubies Sapphires