[Broadway_Alien]

Hi,
Why are diamonds brittle? An ancient scientist - I think it was Pliny the Elder - said that if you hit a diamond as hard as you can on an anvil it would not break. But I'm told otherwise. I thought diamonds were the hardest substance known to man.
Has it got to do with something along the lines of its molecular structure?
Thanks,
Broadway

[Donaldson Tan]

not all structures are perfect. i believe diamond will crack if u hit it along its fault line

[Froggirl]

Hardness (ability to resist scratching) should not be confused with toughness (resistance to chipping or breaking).

While diamonds are the hardest gems in the world they are not very tough. Their molecular structure consists of plains that can be split apart relatively easily if you hit them at the right angle.

So while a diamond can only be scratched by another, there are many things that could chip or shatter it.

[jdurg]

Remember, diamonds have a distinct, crystalline structure of carbon atoms.  Carbon is a non-metal.  Non-metals bind to each other via a crystal structure, or haphazardly.  As a result, a strong force applied to it will cause the structure to breakdown, and as a result the item will shatter.  (Glass is a good example of an amorphous substance.  It doesn't really have any type of crystal structure to it, but will easily shatter if force is applied).  

For metals, they are composed of the metal nuclei surrounded in a sea of electrons.  Therefore, when you hit the metal hard, you just move the nuclei around but they are still in that 'sea of electrons'.  As a result, the metal doesn't break.

[Mitch]

People cut diamonds, don't they?

[Froggirl]

They do cut diamonds, but its typically with other diamonds  

[Mr. Pink]

Diamonds shatter very easily. My father sometimes breaks poorly-cut or low quality diamonds if he tries to set them in, say, a ring. It is a bit tougher than glass, and can scratch any substance known to man, but shatters about as easily as a quartz crystal. Diamond stone cutting cant be done by anyone, and even the toughest, highest-quality diamonds can be broken in the hands of an amature gemcutter.

[Broadway_Alien]

Thanks everyone for your replies.
But there is still one thing I don't understand... the carbon atoms in the diamond crystal are covalently bonded - doesn't that make the bonds fairly strong? I know that in ionic compounds, when a force is applied to one part of the lattice, the force of repulsion between the now-aligned like charges forces the lattice apart.
Is this in any way similar to how diamonds are brittle?

[Mitch]

Bond strength is too complicated to get into and has been covered in past threads. Groggirl answered your question pretty well.

[AWK]

Cleavage planes in diamond are exactly perpendicular to covalent bonds

[Froggirl]

Groggirl...hmmm...I like it  

[Mitch]

Sorry Froggy.

[Broadway_Alien]

Okay, thanks everyone.