Loose diamonds are characterized into 4 c's:-

1.  Clarity
2. Carat
3. Color
4. Cut

• Clarity refers to the internal flaws & inclusions & are sub divided into clarity ranging from if (internally flawless to i1 (inclusion visible from naked eye)
• carats refers to the weight in carats of a specific diamond, 1 carats diamonds is approximately 0.200 milligrams and on the other hand one cent is equal to 0.01 of 1 carats
• Color refers to the shade of the diamond under pure white background, for instance the purest & whitish color of a diamond starts from "D" and then E,F,G,H,I,J,K,L,M,N,O,P" colors after J are going closer to yellow & colors which are between N,O,P are mostly Fancy yellow diamonds & are sometimes even more expensive than the whitest of the white colors
• cut represents the measurements of the table, density, pavilion of a diamonds, cut can be characterized into "excellent" "very good" "good" "fair" & along with cut even diamond florescence is an important aspect during selecting a diamond. 

You can visit Eva Gems, if you are interested in Wedding Rings with Diamond necklaces.