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Name: Mary Beth
Status: student
Grade: K-3
Location: NY
Date: November 2007

Question:
I bought my 8 year old nephew a lab created diamond (I was told that it was lab created and it was not very expensive). It did cut glass so I had no reason to think that it was a lab created diamond. My nephew is a little scientist and decided to do some experiments. In his last experiment, he threw the diamond into a campfire to see if it would burn. It did not burn, but it kind of shattered from the inside out, but maintained its outward shape. My nephew wants to know if that is what a diamond would do, or if that is proof that it is glass or something else?



Replies:
Unfortunately, true diamonds do degrade and darken, in fires, and in extreme heat and air they can burn completely away. Shattering seems plausible too. But I am not an expert on jewels.

Diamonds are pure carbon, so they can react to make CO2 gas and leave no noticeable ash or residue. But this requires a fairly hot fire (such as in the orange-glowing ash core of a fire) plus free oxygen (not always very present in the core of a burning pile).

In the absence of oxygen, they just turn to graphite. In the early stages of this transformation, the diamond crystal gets dark inside due to heat-generated defects or tiny growing nuclei of graphite embedded inside. This graphite is a little less dense than diamond, so growing seeds of it may well push the diamond outward from the inside, until the diamond cracks from the inside outwards.

It is vaguely possible that a lab diamond has within it some of the metal that it was grown from, either as tiny inclusions, or dissolved colorlessly in the crystal lattice. If dissolved impurities precipitated in the heat, that might help cracking too.

If any crystal (diamond or other) is suddenly immersed in a hot environment, so that the outer parts heat up significantly faster than the inside, the outside would expand putting the interior in tension. This too would cause cracking like you saw. But for a tiny 1 carat diamond tossed into the hot-but-lazy gas inside a fire, it seems to me the thermal assault would usually be slow compared to the diamond's typical fast heat-transfer rate and small size. What would crack a diamond would be more like plunging into molten metal or a rocket flame. So I am not sure this cracking would happen to a diamond in a campfire. This cracking tends to hint that you have an oxide crystal such as zirconia, which has a much lower thermal conductivity than diamond. But I do not know for sure.

You did not mention whether there were discoloring effects. Complete lack of darkening might tend to imply a non-diamond too.

If it scratches glass, it is not a glass. I do not think there are any extra-hard oxide-based glasses. Crystals can be harder. Many oxide-based crystals are a bit harder than glass. The diamond substitute Moissanite is silicon carbide crystal, which will scratch glass too. It is not an oxide and has high thermal conductivity almost like diamond. It is about half as expensive as diamond, much more expensive than zirconia.

I read on-line that some jewelers have a tester called a thermal conductivity probe. This can clearly distinguish between:

- diamonds or Moissanite (high conductivity), and

- oxide crystals such as zirconia (lower conductivity).

It might be inexpensive to ask for, and it mimics what the fire probably did (thermal conductivity test), but with better calibration and no destructive effects. It might resolve your interesting question.

Jim Swenson


Mary Beth,

I do not know if the shattering can prove anything apart from the fact that there must have been a crack or fissure within the object or some trapped gas. When heated, the fissure can cause disproportionate expansion of the material which will cause the material to shatter. Likewise, a bubble of gas within the object will expand rapidly on heating and cause the material to shatter.

I am not familiar with the properties of lab-created diamonds, but maybe someone else in this forum can respond to that.

Greg (Roberto Gregorius)


You have a very observant nephew. Encourage that. The problem with the experiment is one of multiple processes going on. "In principle" diamond (carbon) will react with oxygen to "burn" forming carbon dioxide (CO2). However, placing the sample in a campfire allows other competing processes to occur that are faster than breaking the carbon-to-carbon necessary to result in "burning" forming CO2.

In order to "burn" at a rate fast enough to observe you would have to reach a certain minimum temperature. However, at lower temperatures, the diamond develops differences in temperature between the "inside" and "outside" that are large enough to cause the diamond to shatter before the chemical reaction can occur. If you are careful you could see the same thing happen by heating a piece of glass to high temperature, and plunging it into cold water. The stresses are so large that the "chemical" reaction is far slower than the expansion/contraction of the glass itself. As a result, the reaction that "should" occur is overwhelmed by the fast temperature change that causes the glass to shatter before the slower reactions can happen.

The lesson for your nephew that I see is that observing the "process" that is happening is as important, or maybe more important, than the actual result. Observing the "unexpected" is how science progresses. So don't be disappointed that the "expected" result did not occur.

Vince Calder



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