The quality of a diamond is determined by four factors commonly known as the "4 Cs" -- Carat weight, Clarity, Color, and Cut.
Carat	1 carat = 200 milligrams = 100 points
Color	For a diamond to be considered "colorless," the G.I.A. requires that it be a D, E, or F.  However, the D-Z scale is continuous, so the difference between an F and G is very small.  The average color for engagement diamonds in the United States is G to H.
Clarity	There are several grading systems used to describe clarity.   By far, the most popular is the Gemological Institute of America's (G.I.A.) scale, which ranks diamonds as Flawless (FL), Internally Flawless (IF), very very slightly included (VVS), very slightly included (VS), slightly imperfect (SI), and imperfect (I)
Cut	http://www.diamondreview.com/tutor/4cs/tutor_4cs_cut5.shtml
Shape	Round  Family-centered, dependable, unaggressive. Oval  Individualistic, creative, well-organized, willing to take chances. Heart  Sentimental, feminine, sensitive, trusting. Rectangle/Square   Disciplined, conservative, efficient, honest. Pear  Conforming, considerate, adaptable. Marquise  Extroverted, aggressive, innovative, career-centered.
Diamond G.I.A Code and Price:
Carat	shape	color	clarity	cut	top 10%	top 25%	Average	lower 25%	low 10%
0.25	Round	D	FL	VG	$1,290.00	$1,290.00	$1,290.00	$1,290.00	$1,290.00
0.25	Round	E	FL	EX	$1,189.00	$1,189.00	$1,189.00	$1,189.00	$1,189.00
0.5	Round	D	FL	EX	$6,698.00	$5,931.00	$4,149.00	$2,367.00	$1,600.00
0.5	Round	D	IF	EX	$5,961.00	$5,257.00	$3,682.00	$2,089.00	$1,403.00
0.5	Round	D	VVS1	EX	$5,727.00	$5,068.00	$3,537.00	$2,005.00	$1,346.00
0.5	Round	D	VVS2	EX	$5,400.00	$4,778.00	$3,333.00	$1,888.00	$1,266.00
0.5	Round	E	FL	EX	$6,458.00	$5,717.00	$3,997.00	$2,276.00	$1,535.00
0.5	Round	E	IF	EX	$6,316.00	$5,591.00	$3,908.00	$2,225.00	$1,500.00
0.5	Round	E	VVS1	EX	$6,079.00	$5,381.00	$3,760.00	$2,140.00	$1,442.00
0.5	Round	E	VVS2	EX	$5,748.00	$5,088.00	$3,554.00	$2,020.00	$1,360.00
0.5	Round	F	FL	EX	$6,101.00	$5,400.00	$3,770.00	$2,139.00	$1,438.00
0.5	Round	G	FL	EX	$5,572.00	$4,928.00	$3,432.00	$1,936.00	$1,292.00
0.75	Round	D	FL	EX	$11,078.00	$9,804.00	$6,846.00	$3,887.00	$2,614.00
0.75	Round	D	IF	EX	$10,912.00	$9,657.00	$6,743.00	$3,828.00	$2,574.00
0.75	Round	D	VVS1	EX	$10,634.00	$9,412.00	$6,571.00	$3,730.00	$2,570.00
0.75	Round	D	VVS2	EX	$10,246.00	$9,068.00	$6,330.00	$3,593.00	$2,414.00
0.75	Round	E	FL	EX	$10,692.00	$9,463.00	$6,603.00	$3,754.00	$2,525.00
0.75	Round	E	IF	EX	$10,525.00	$9,316.00	$6,505.00	$3,695.00	$2,485.00
0.75	Round	E	VVS1	EX	$10,248.00	$9,070.00	$6,334.00	$3,597.00	$2,419.00
0.75	Round	E	VVS2	EX	$9,860.00	$8,727.00	$6,093.00	$3,460.00	$2,326.00
0.75	Round	F	FL	EX	$10,103.00	$8,942.00	$6,264.00	$3,549.00	$2,388.00
0.75	Round	G	FL	EX	$9,367.00	$8,289.00	$5,784.00	$3,280.00	$2,202.00
1	Round	D	FL	EX	$21,152.00	$18,714.00	$13,048.00	$7,382.00	$4,944.00
1	Round	D	IF	EX	$20,701.00	$18,315.00	$12,771.00	$7,227.00	$4,840.00
1	Round	D	VVS1	EX	$19,948.00	$17,649.00	$12,308.00	$6,967.00	$4,668.00
1	Round	D	VVS2	EX	$18,895.00	$16,718.00	$11,661.00	$6,604.00	$4,427.00
1	Round	E	FL	EX	$20,298.00	$17,959.00	$12,525.00	$7,090.00	$4,751.00
1	Round	E	IF	EX	$18,579.00	$16,439.00	$11,466.00	$6,494.00	$4,353.00
1	Round	E	VVS1	EX	$17,957.00	$15,888.00	$11,083.00	$6,278.00	$4,210.00
1	Round	E	VVS2	EX	$17,086.00	$15,118.00	$10,548.00	$5,977.00	$4,010.00
1	Round	F	FL	EX	$18,952.00	$16,769.00	$11,696.00	$6,623.00	$4,439.00