If diamonds are made of carbon? and coal is made out carbon

Sammyg

If diamonds are made of carbon? and coal is made out carbon, why are diamonds transparent and coal is not transparent?

Phrak

Crystaline structure?

Division

Diamonds are carbon put into high amounts of pressure and heat.

The whole process is very expencive, too expencive for any practical use in making them. The quantity of diamond you get out of any amount of carbon is too costly to be beneficial, therefore, this process is rarely used, if ever.

ZapperZ

What Phrak said. It is the crystal structure that is different, i.e. how the carbon atoms are arranged.

Note that it isn't just the optical properties that's different due to the crystal structure difference. The physical strength of the material is also different. Compare the hardness of graphite versus diamond.

This is a good example on why many of the properties of solids depends NOT on the "atomic" properties of the individual atoms making up the solid, but on the "collective" property of the atoms, i.e. how they behave as a whole group. This is why how the atoms of the material are arranged and how they behave collectively are extremely important.

Zz.

Bob S

Diamond density is about 3.5 to 3.53 grams per cubic cm, while graphite is only 2.0 to 2.25 grams per cubic cm.

Vanadium 50

And what does that have to do with transparency?

Bob S

[Bob S] Diamond density is about 3.5 to 3.53 grams per cubic cm, while graphite is only 2.0 to 2.25 grams per cubic cm. Diamond is one of several carbon allotropes, which include fullerenes {buckyballs), nanotubes, graphite, graphene, and coal (not really an allotrope). Graphite is a good electrical conductor, while diamond is a good insulator. Diamond is a very good thermal conductor but at the same time an electrical insulator (doesn't obey the Wiedeman Franz law). If sulfur can have red, yellow, and black allotropes, why can't carbon have one that is transparent?

Phrak

Yes, but the industry produces in the region of 3 billion carats, or 600 metric tons of synthetic diamonds a year.

f95toli

Indeed, diamonds are much more common that most people think; the reason you don't see more often them is simply that most of the diamonds are quite small (e.g. the ones used on tools such as drills etc).
That said, there are "simple" methods that can be used to grow quite large diamonds but the ones I've seen have all been almost pink so they weren't exactly something you would make jewelry of; but they are still as hard as a "proper" diamond so they have their uses.

There are some companies that can grow large, transparant, diamonds that can be used in jewellry; the ones from e.g. Apollo Diamond are apparantly very good.

There are also companies that are trying to make large diamond substrates for e.g the semiconductor industry. Large diamonds can actually also be used as a substrate for growing YBCO thin films (a high temperature superconductor) although there aren't many people doing that

v2kkim

Maybe we can explain using semiconductor concept. Semiconductor has a band gap which is the difference between valence band and conduction band, and photons of larger energy than this band gap can be absorbed. In diamond the band gab is larger than visual light I think, but in silicon it is not. So silicon can be used as a infrared light pass filter blocking visual light. Coal is obviously mixture of many crystal like forms so the overall band gap is almost zero, meaning that all frequency light will be absorbed.