High-quality Diamonds are considered to be one of the most powerful Jyotish gemstones. 
Color: Choose a colorless or near colorless Diamond. Color grades D-E-F are colorless; G-H are near colorless. These are the most desirable colors.
Clarity: Choose clarity grades of VS1 or better: this means very minor inclusions that are difficult to locate by a trained Diamond observer with a 10X power loupe.
Shape: Round, Emerald cut, Princess cut, Radiant cut, or Oval.
Size: 1.00 carat or larger. Most people do not need two carats or larger.
Certification: Gemological Institute of America, GIA, certified Diamonds are preferred.
Beware: Avoid black inclusions, even minute ones at 10X. Also avoid: extremely thin or very thin or thick girdles; medium to strong blue fluorescence; fair to poor symmetry; overly deep Diamonds, which are dark in the center; and any laser-drilled or filled Diamonds.