Eli5 Why are diamonds clear when they are made of carbon?

Question

Diamonds are clear and see-through yet they are made of highly pressurized carbon which if you see in its unpressurized state looks like black powder or a handful of ash, What happens to cause it to become see-through like that. For that matter the same question for sand and silica into glass.

In: 13

Answer 1

The color as well as the energy of light is determined by its wavelength. When light hits a molecule, the light can be absorbed and its energy transferred to one of the molecule’s electrons, putting it into a higher energy state, but only if the energy of the light is exactly equal to the difference in energy between the states of the electron. Therefore, certain molecules can only absorb certain energies, i.e. colors, of light.

In a diamond, all the carbon atoms are arranged into a neat crystal structure where each carbon atom has four single bonds to four other carbon atoms ([see here](https://www.chemguide.co.uk/14to16/structure/diamond.gif)). The electrons in these single bonds are very happy where they are and require a lot of energy to be put into a higher energy state. The energy of light increases as its wavelength decreases and therefore only invisible ultraviolet light has enough energy to be absorbed by the electrons in a diamond. Visible light can not be absorbed and therefore a diamond is clear.

Other forms of carbon are black because they have bonds where the electrons are easier to excite to a higher energy state. In graphite ([structure of graphite](https://qph.cf2.quoracdn.net/main-qimg-54f68b376f67539dbfa28910d245c65d-pjlq)) for example, there are lots of double bonds between carbons, and these double bonds create a system of easily excitable pi electrons that can absorb many different energy levels. All wavelengths (=colors) of light can be absorbed and when every color of light is absorbed, the substance appears black.

Coal is similar. Coal is mostly amorphous carbon, i.e. carbon atoms bound together without any global structure. There are also double bonds and “dangling” electrons that can be excited by different visible wavelengths of light and therefore amorphous carbon also absorbs all colors and appears black.