ideal - ID - excellent - EX

ideal，ID：完美
excellent，EX：理想
very good，VG：非常好
good，G：好
fair，F：一般
poor，P：差

For the most beautiful diamond, look for a symmetry grade of excellent (EX), very good (VG), or good (G) for a GIA graded diamond, and ideal (ID), excellent (EX), very good (VG), or good (G) for an AGS graded diamond.

American Gem Society，AGS：美国宝石协会
gem [dʒem]：n. (经切割打磨的) 宝石，难能可贵的人，风景优美的地方，美妙绝伦的事物 adj. (珠宝) 最佳品质的 v. 用宝石装饰
Gemological Institute of America，GIA：美国宝石学院
symmetry ['sɪmətri]：n. 对称，相似，相仿，相等

References

https://mdcdiamonds.com/Education_Cut.cfm