When shopping for a diamond, most people focus on the “4 Cs”—cut, color, clarity, and carat. But what many don’t realize is that cut is the most important factor in how brilliant and beautiful a diamond appears. It’s not just about shape—cut refers to how well a diamond’s proportions, angles, and facets are executed to interact with light. The better the cut, the more your diamond will sparkle.
Light performance is the key. When light hits a diamond, it reflects off the surface, enters the stone, bends (a process called refraction), and ideally reflects internally before exiting back through the top. A well-cut diamond captures and redirects this light efficiently, maximizing its return to the eye. This is what creates brilliance (white light), fire (colored flashes), and scintillation (sparkle when the diamond moves).
If a diamond is cut too deep, light leaks out of the bottom, making the diamond appear dark in the center. If it’s too shallow, light escapes through the sides, causing a dull look. Only a diamond with ideal proportions—proper table size, crown and pavilion angles, and overall symmetry—will reflect light correctly and give you that stunning, lively appearance.
Each diamond has a series of flat surfaces called facets, which act like mirrors or prisms for light. A standard round brilliant cut diamond has 57 or 58 facets arranged in a very specific geometry. The largest facet, the table, is the window into the stone, while the crown and pavilion facets work together to bend and reflect light. Even slight misalignments in these facets can create “dead zones” that appear dark, reducing overall sparkle.
The Gemological Institute of America (GIA) assigns cut grades for round brilliant diamonds ranging from Excellent to Poor. An Excellent cut diamond has ideal proportions and symmetry, returning the most light and showing exceptional brilliance. On the other hand, a diamond with a Good or Fair cut might look okay on paper but appear noticeably dull or lifeless when viewed in person. That’s why cut should never be compromised, even if you’re balancing carat weight or clarity.
Fancy shapes like ovals, pears, emeralds, and cushions don’t always come with a standardized cut grade. For these, visual performance—how the diamond handles light—becomes even more important. For example, some ovals show a “bowtie” effect if they aren’t cut well, creating a dark shadow across the center.
At Johnston Jewelers, we go beyond the lab report. With over 50 years of experience, we hand-select diamonds not just by certification but by how they perform in real-world lighting. We explain how cut proportions, angles, and facet patterns affect brilliance, and we compare stones side-by-side so you can truly see the difference before you choose. Our goal is to help you find a diamond that doesn’t just check boxes—but one that comes to life when it’s worn.