A Simple Guide to Buying a Round Diamond

Don’t let a jeweler mislead you by overcomplicating the buying process.  Keep it simple: there are only a few things you need to keep in mind when buying a round diamond.

  • Maximize cut, carat, color, and clarity in that order.
    • Cut is most important because it determines how a diamond shines. There’s a lot of variation in cut quality, even within GIA Excellent.  Use the cut estimator to find the best cut stones.
    • Clarity is least important because it only needs to look clean to the naked eye.
  • Use the diamond price curves to find the lowest price diamond that matches your criteria. I’ve highlighted the best cut stones that qualify as GIA Excellent and AGS 0 Ideal to make the search process as smooth as possible.

The round brilliant is the most straightforward diamond shape to buy.  It is a standardized 58 facet structure that has been extensively researched, making it the only shape that can be bought “by the numbers.”  The main idea is: don’t pay for what you can’t see.

Cut

Maximizing cut accomplishes two things:

  • Cut determines light performance, which improves all other factors. A bright, flashy stone will look bigger, hide color better, and make it harder to see inclusions.
  • A well-cut stone ensures that the carats are distributed optimally, meaning you won’t be paying for carats you can’t see. Think of it like an iceberg – if a diamond is too deep, then it will look smaller because its carat weight is hidden beneath the surface.

A round diamond’s geometry is dictated by its table %, crown angle, and pavilion angle.  The AGS and GIA proportion-based cut grade criteria use these three values to assign a cut grade.

AGS’ cut grade is an objective measure of light performance based on computer-simulated light ray tracing.  GIA’s cut grade is based on based on aggregated consumer preferences for different proportion combinations.  If two grading labs (one based on computer simulation, one based on consumer opinion) both agree that a stone is at the top of their scale, then it should be a brilliant stone.  The cut estimator shows how the two labs line up for a given combination of proportions.

The importance of proportions is shown below.  Both diamonds below are GIA Excellent diamonds, but some GIA Excellents are better than others.

  • The first is the archetype of what you should be looking for.  It has ideal proportions, with table % crown angle, and pavilion angle that qualify it as AGS 0 Ideal.  It has excellent optical symmetry.  You can clearly see the 8 arrows radiating from the center in the face-up position.
  • The second is an example of what to avoid.  Even though it’s GIA Excellent, it has worse proportions that would only qualify it as AGS 3 Good.  The arrows have disappeared, which leads to less contrast and brilliance.

Buy A Diamond That Looks Like This

Avoid A Diamond That Looks Like This

Carat/Color

Most people choose to maximize carat over color because size is readily apparent, but color is harder to see.  Since color differences are less noticeable, buy the lowest color that still looks white to you.  This is usually around H or I for most people; some higher end J’s also offer good value.  Of course, follow your own priorities; if you want a D, get a D and then get the biggest stone that fits your budget.

A secondary consideration for color is fluorescence.  It’s often repeated that fluorescence can potentially improve color.  In theory, this makes sense because light is an additive color system where blue (fluorescence) + yellow (diamond color) = white.  However, in practice most viewing conditions (e.g. indoor lighting) do not produce the UV light that activates fluorescence.  Since fluorescence is not observed most of the time, you shouldn’t rely on it to improve color.

The market typically discounts stones with fluorescence, so you can take advantage of this to get a good deal.

Clarity

VS1 will be eye-clean – you will not be able to see any inclusions with the naked eye.  Higher clarity is unnecessary; you won’t be able to see the difference between VS1 and VVS2 or higher, so only get the higher clarity if it’s a good deal (i.e. you’re getting the extra clarity for free).

To save money, you can find eye-clean stones in lower clarities like VS2 and SI1, but you’ll need to evaluate each diamond on a case by case basis.  When searching for eye-clean stones in the VS2 and SI1 ranges, avoid black crystals in the middle of the table.  Instead, try to find stones with inclusions near the edges that can be hidden under a prong.

Conclusion

Using these guidelines, you should be able to find a beautiful diamond that provides the most bang for your buck.  I’ve tried to make the search process as smooth as possible by highlighting the best cut stones on the diamond price curves.  Good luck!