Crystal refers to the ‘crispness’ of the crystalline substance of a diamond (or any other gemstone for that reason) . It usually has no direct effect on the actual clarity grade assigned to a particular diamond and cannot simply be deduced by looking at the diamond’s respective grading report.
Rather, when gem dealers talk of crystal, they refer to a subset of clarity which scientifically speaking takes account of the orderly arrangement of the material of a particular diamond at a (sub)microscopic level, a quality that nonetheless becomes easily visible to the discerning eye.
The most astounding examples of diamonds of the finest crystal are those belonging to the legendary true old Indian Golconda Type IIa diamonds. When compared to modern day counterparts of various origin, possessing otherwise the same caveats, Old Indian stones simply outrival the first! What sets authentic Golconda diamonds apart is that they are extremely limpid! This is expressed by the way light penetrates them and how we as humans perceive their transparency. A superior crystal always infers that a diamond experienced sublime growing conditions.
Interestingly, when it comes to other coloured gemstones, a superior diaphaneity-as it is also termed- helps to alleviate visual extinction and reduces turbidity in a gem.