Symmetry is graded as Excellent, Very Good, Good, Fair or Poor on a GIA report. When polishing a rough diamond the aim is to cut the heaviest, most valuable diamond possible. This often means polishing a diamond with imperfect symmetry to avoid inclusions or just leaving more weight to achieve a "magic weight" (like 1.00ct). The polished diamond may be slightly off round, have variations in girdle thickness, tilting of the table, and off centreing the table or the culet etc. Often symmetry defects in a diamond are the result of great skill, rather than an indication of poor skills.
The diamond images we have used in this tutorial are mostly symmetrical. But in the real world very few diamonds are perfectly symmetrical, but symmetry is less important to the overall beauty of a diamond than the critical facet proportions. You may never notice any difference between diamonds with excellent or ideal and very good or good symmetry.
The Ideal-Scope reference chart is an effective way to judge symmetry because the patterns we observe with it display "optical" symmetry. Another category of high symmetry diamonds is Hearts and Arrows.
Very thick, medium, and very thin girdle