DIAMONDS — LOGIE 375 



a localized positive hole which can move by reciprocal motion of 

 thermally activated electrons in the valence band, and also contributes 

 to the current. If the source of illumination is removed, the photo- 

 conductivity dies away by the recombination of electrons and holes. 

 A certain amount of energy must be given up when this recombination 

 occurs and may take several forms. One possible form is an increase 

 in the temperature of the diamond. Another possibility is that when 

 the electron and hole recombine, they may do so with the emission of 

 visible light. This is spoken of as luminescence, but in pure diamonds 

 this process will not occur. 



If the light falling on the diamond has a wavelength in the ultra- 

 violet, the electrons will acquire energy from the beam of light. We 

 say that the light has been absorbed and that the diamond has an 

 absorption band in the neighborhood of 2,200 A. No absorption will 

 occur for wavelengths longer than this, and no absorption in the visi- 

 ble part of the spectrum is anticipated. This means that if we start 

 with white light, it should remain white after passing through the 

 diamond. We are thus led to expect that all diamonds should be 

 pure white, with no absorption bands to produce color in the visible 

 part of the spectrum. 



A third method of producing conductivity is by bombarding the 

 crystal with small but energetic particles like electrons or alpha par- 

 ticles. Wlien these impinge on the crystal, they collide with the elec- 

 trons already present and remove them from their covalent bonds. 

 As before, conduction by electrons and holes becomes possible. If the 

 source of the energetic particles is a radioactive material, then each 

 alpha, beta, or gamma ray of sufficient energy may release several 

 hundred electrons into the conduction band, and the effect is a short- 

 pulse of current for each incident particle. 



The following properties of the diamond are expected on the basis 

 of the above reasoning: it should be an insulator; it should be pure 

 white ; and it should show no luminescence and no photoconductivity 

 for wavelengths greater than 2,200 A. The actual behavior is in strik- 

 ing contrast to these predictions. We know that diamonds often are 

 colored, that some of them do show luminescence, that they are some- 

 times conductors, and that they do have photoconductive properties 

 when illuminated by white light. In short, if there is one fact which 

 is very soon apparent to anyone investigating the properties of the 

 diamond, it is that each stone seems to have a quality of its own. Some 

 attempt has been made to group certain types of diamonds together 

 according to their properties. This scheme is helpful, but there is 

 no sharp boundary between one group and another. The first classifi- 

 cation divides diamonds into type 1 and type 2, according to whether 



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