How are the single crystals judged?

A single crystal can be judged on the basis of its quality or a combination of mass and quality. Judging criteria are given below. You can provide different categories, e.g. according to age, best quality, best school, longest crystal etc...

Judging criteria

Credit: picture by Dirk Poelman
Single crystals can be judged on the basis of combining mass and quality factors as outlined below.

The quality is judged by experts who will rank the crystals on a scale of 0 to 10. A score of 10 will be given to a perfect gem-quality crystal that fits the ideal crystal structure known for the chemical. 

  1. The crystal is weighed, and the mass M recorded. It is possible to require a minimum mass, e.g. the crystal must be a minimum of 0.5 g to be eligible for judging.
  2. The quality of the crystal is judged on a scale of 1 to 10, with 10 representing a perfect crystal. The following factors will be considered in judging quality: 
  • match/mismatch with crystal type (out of 2)
  • presence/absence of occlusions (out of 2)
  • intact/broken edges (out of 2) 
  • well formed/misformed faces (out of 2) 
  • clarity/muddiness (out of 2)
    Total Observed Quality Q = x.xx (out of 10)
  1. The Total Score is then determined as follows:
    Total Score = [log (M+1)] x Q
    The logarithm of the mass is chosen so that large poor quality crystals don’t swamp out smaller good quality crystals.

    The value 1 is added to the mass so that crystals weighing less than 1 g get a positive score.

Overall score

Instead of using the Total Score to indicate the winning crystals, one can also take the amount of starting material into account.  

A 100 per cent yield crystal made from 100 g (Mt) that scores a perfect 10 on quality (Qt) would get a theoretical maximum of: 

[log (100+1)] x 10 = 20.01 

The actual score can also be expressed as a percentage of the maximum. The crystal with the highest Overall Score is then the winning crystal. 

100 x {[log (M+1)] x Q} / {[log (Mt+1)] x Qt} = Overall Score % 

For example, the best overall crystal in the  Canadian 2001 contest with 150 g starting material weighed 46.53 g and had a quality of 8.65. Its overall score was: 

100 x {[log (46.53+1)] x 8.65} / {[log (150+1)] x 10} = 66.6%

This score is nearly an absolute score that could be used to judge different types of crystals grown from differing amounts of starting material.