Why are gemstones clear, while rocks/metals aren’t?


I think it might have something to with the crystalline structure, but you can have metal crystals and those aren’t clear.

In: Physics

Depends on how the crystals form as in speed/temp/location

That’s all the information I have, I’m sorry.

Mineral/rock/metal are scientific terms while gemstone is more of a cultural term. A gemstone is a rock or mineral that people use in jewelry. Most people like having clear and shiny gemstones, but there are some opaque gemstones like pearl, turquoise, and lapis lazuli.

There are some clear minerals that you rarely see as gemstones. Calcite is perhaps the most common example. If I had to guess, the biggest problem with calcite is that it has a low hardness value, which means that it is really easy to scratch. (For example, I’m pretty sure that the metal from the ring could scratch a calcite stone.) Gemstones like diamonds, rubies, emeralds, topaz, quartz, etc., all have really high hardness, which makes them resistant to scratching. There are probably other reasons too.

The color of a stone and whether light passes through it is related to the chemical structure of the crystal. Impurities in the crystal can actually add color to an otherwise clear stone. ~~For example rubies, sapphires, and emeralds are all the same type of mineral (corundum) with different types of impurities. It is possible for a corundum stone to have too many impurities and be too opaque for use as a gemstone.~~ Impurities are also why there are different colors of diamonds, which are normally clear.

The way an object interacts with light fundamentally comes down to it’s electrons and how they are structured as these are the particles which interact with light.
For a photon of light to be absorbed it needs to interact with an electron and give it its energy. It turns out that electrons in atoms can only take certain values of energy and not values in-between these.

In the simplest case we have hydrogen gas with it’s 1 electron in a certain orbital around the proton, when you pass light through this hydrogen gas it can only absorbs wavelengths of light corresponding to the difference in energy between its orbitals.

For solid materials this is much more complicated but the basic physics is the same, for light to be absorbed it needs to correspond to a transition in electron energy which is allowed by the material. For transparent materials like glass and gemstones there are no transitions which correspond to the wavelengths of visible light that can pass through it.
For crystals which aren’t transparent that means there are transitions allowed which absorb visible light.

Metals on the other hand are somewhat special in that they exist as positive metal ions surrounded by a ‘sea’ of delocalized electrons, in contrast to something like diamond which is made of carbon atoms connected by covalent chemical bonds.
Because of this sea of electrons a metal can interact with basically any photon of light, these photons are absorbed and then almost instantly remitted which results in almost all colours of light being reflected by the metal.