SIR - In 1955, some of us announced the first reproducible synthesis of diamond1, details of which were subsequently published2. These results marked the be-ginning of the present synthetic-diamond industry. But from the outset there were doubts in our team as to whether the first diamond grown by our technique (which we will call the run 151 diamond) was truly synthetic at all, or whether it was instead just a fragment of a natural diamond seed that got into the experi-ment inadvertently. We have now re-analysed the run 151 diamond using modern spectroscopic techniques and have found that it is indeed a small piece of a natural type la diamond.

this spectrum shown in the figure resem-bles that of a natural nitrogen-containing type la diamond4. In particular, there are coincidences of the absorption bands at about 1,365 cm"1 (related to nitrogen platelets), 1,330 cm"1 (a Raman frequen-cy, rendered infrared-active by defects and impurities), 1,280 cm"1 (from nit-rogen in the 'A' aggregate form) and 1,175 cm"1 (from nitrogen in the 'B' aggregate form). We also show the spectrum of a typical nitrogen-containing synthetic type Ib diamond, which has characteristic bands at 1,130 and 1,343 cm"1 (ref. 5); neither of these bands is seen in the run 151 diamond. We conclude that the run 151 diamond is a small piece of a natural type la diamond.

How the natural diamond got into the run 151 experiment is not clear, although it came to light only a week later, when the iron pellet from the run was being polished for metallographic examination. After we found this diamond and took it to be synthetic, Hall used a similar synthetic system of iron/iron sulphide/graphite in his 'belt' apparatus, which used a carbide piston and cylinder to achieve high

pressures6. This led to further successful

runs and ultimately to the development of

the process for synthesizing diamonds at

high pressures and temperatures from

graphite reacted with molten group VIII

metals and alloys, which we described

fully in 19592 (after a US Department of

Defense secrecy order had been lifted).

Our mistake was therefore clearly a most

serendipitous one, as it provided the

impetus to experiment with that system

at higher pressures, leading quickly to

the "right" and "reproducible" results.

figure a