Ask A Scientist General Science Archive

name         Linda
status       other
grade        other
location     DE

Question -   What is a "simple" explanation of visible carbon (black
specks) in a diamond?
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High temperature and pressure is necessary to form "natural" diamond (that
is, geological diamond) from graphite. Geological conditions are not
necessarily entirely uniform, which results in small specks of unconverted
graphite. Those are the tiny black specks. Larger inclusions of graphite
also occur, but these samples don't make it to the gem market. They would
be used in industrial applications where appearance is not important.

Vince Calder
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Hi Linda!
See both diamond and  carbon black  are allotropic
substances made of carbon atoms. What made them so
different is its molecular structure, meaning
the way the carbon atoms are connected and arranged
in space,  different in both cases.
The so called "specks" found in diamonds are only
defects or imperfections, due to inclusions of
small pieces of black carbon inside a diamond
structure.
Thanks for asking NEWTON!

Mabel
(Dr. Mabel M.Rodrigues)
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Easy -  Carbon can be diamond or graphite.  Or any mix.
Eventually diamond can turn back into graphite.
Specks of graphite can be embedded in diamond.
It is just a question of how it is made.
In natural diamonds it is a matter of luck that it gets made without such 
defects.

Some diamond is so impure with the occasional graphite-like clumps of atoms
that it gets jet-black (like dark glass) instead of shiny gray like pure 
graphite.
Mixtures of several degrees are possible.

When a diamond is growing, or after, if there is some metal impurity 
trapped in one small place,
it can catalyze the diamond in that spot to turn into graphite.

If a diamond gets very hot it turns entirely to graphite.
So, heating a diamond above a few hundred degrees C (not hot enough to 
convert much),
might start graphite growing around tiny previously invisible defects in 
the diamond.
I wonder if surviving a fire often does this to diamonds.

It is also presumably possible to make graphite specks inside a diamond by 
focusing a pulsed laser in there.

Jim Swenson
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