Short answer, post your wants and budget over on Pricescope-Rocky Talk, the diamond nerds will happily help you.
Very Long Answer:
You will get a much better deal with far less risk buying online since you can select for all the variables. Reputable dealers are JamesAllen, BlueNiles, Whiteflash, HighPerformanceDiamonds, Brian Gavin, B2C. WF and HPD have the best return and upgrade policies of these.
Look for GIAXXX or AGS0 (GIA is tougher on color, AGS on cut)
table – 54-58% (55-57, more fire)
depth 60-62.3% (59, sometimes)
crown 34-35 degrees (34.5, ideal)
pavilion 40.6 – 41 degrees
Run all potential stone through the HCA Tool (https://www.pricescope.com/tools/hca). You want to see Excellent on light return, fire, and scintillation, and Very Good or Excellent on spread. An overall score of 2.0 or lower means a diamond is worth exploring further (but a 1.0 isn’t necessarily better than 1.9).
Anything not eliminated by the HCA is worth exploring. You want to look at an Ideal-scope or ASET to see light return.
Color: This is personal, but most people will ‘see’ tint starting at an I. But, an I or J with strong Florescence (that a gemologist says is not making the stone milky) can ‘lift’ the color of a J.
Clarity: VS1 & 2 are generally safe. Many SI1 that are very carefully selected can be great.
Cut: If you have to choose between GIAXXX and AGS0, go with AGS0.
Hearts & Arrows (or not): This is simply a cutting pattern (like comparing a jewish star to a 5-pointed star). Being H&A means the stone is cut to have precise angles that yield that pattern. The other way rounds are cut or usually ‘commonBut, this does not mean the stone will have good performance (sparkle, scintillation, brilliance, etc). For that, you need to look at the numbers above. Also, there is not an industry standard definition of H&A — but the super ideal diamonds will meet everyone’s standards.