Does coal not sparkle because of its colour or because light does not refract / reflect from the cut facets? Thanks!
In: Physics
Coal doesn’t sparkle because it’s not a crystalline solid, and it’s not transparent. It has randomly distributed particles so any light that falls on it gets absorbed (because it’s black), or reflected in random directions and difused. Light that falls on a cut diamond gets focused by internal reflection into certain paths, which is why you see “no sparkle-blinding sparkle-no sparkle-again” when you rotate it.
I’m not positive but I’m guessing you could polish coal and it would shine if not sparkle.
Diamonds are different because the material is transparent, you get reflections from the outside facets and reflections from light that passes through those facets and hits other facets. Light is bouncing around everywhere, aka sparkle.
A good diamond cut has geometry that intensifies the effect.
Maybe off-topic but here’s why diamonds are transparent:
A diamond consists of a 3 dimensional array of carbon atoms bound together. A chemical bond is just electrons moving around two atoms binding them together by electrostatic forces. Electrons can absorb light, which raises their energy level, and they loose the energy in form of heat for example. When a compound absorbs light (=the bond electrons absorb light), and that light has a wavelength within the visible spectrum, the compound has a color. When every visible wavelenth gets absorbed, the compound is black.
More tightly bound electrons need more energy (=smaller wavelength of light) to be excited to higher energy levels. Lets compare diamonds and graphite. The electrons in diamond are too tightly bound to the carbon atoms , they only absorb high energy UV light, which is invisible – so diamonds absorb no color, they are transparent. The carbon atoms in graphite are arranged in sheets, and these sheets are bound together by weaker bonds called pi-bonds. These pi-bond electrons absorb light at almost every wavelength in the visible spectrum – so graphite is black. These pi-bonds are also the reason why graphite conducts electricity and diamonds don’t.
Diamonds are unique in terms of forms of carbon in that they are transparent and very optically dense. This means that light slows down a *lot* when it passes through diamonds. The result is that it bounces around the inside of them a lot, and gets sent off in all directions. Other carbon arrangements will happily absorb this light, and would be much less optically dense even if they didn’t, so they could not sparkle.
Diamonds are specific arrangement of carbon atoms that just happen to reflect light really, really well. Other things might be made of carbon, but they dont have the same structure.
Natural, uncut diamonds are a lot less shiny than you might think. Diamonds are cut and polished *very* specifically in order to reflect as much light as possible. Imperfections like cloudiness or inclusions (other minerals trapped inside) or fractures will all make the diamond a little less shiny and sparkly.
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