The size of the table is important because it helps determine how much light enters and leaves the diamond. Table size or table percentage indicates how large the table is in relation to the diameter of the stone. In a round brilliant, it’s expressed as a percentage of the average girdle diameter, which is determined by measuring the girdle in several directions and then adding the smallest and largest measurements and dividing by two. The average girdle diameter is an important figure: It’s used as a basis of comparison for many other diamond dimensions.
Before the invention of the rotary diamond saw, table percentages were as small as 40 to 50 percent. This was mostly because grinding was the only way to shape a diamond. Cutters were forced to form each piece of rough into a single finished stone. A small table allowed the cutter to keep more of the diamond’s rough weight.
When the rotary diamond saw came into use in the early 1900s, cutters found they could get two stones from one well-formed rough crystal. They were able to cut large tables and still retain a high percentage of the original rough weight. After that, cutters began cutting larger and larger tables. Eventually, many diamonds were cut with tables so large that they caused a reflection that overpowered the light from the other facets.
Today, most round brilliant table sizes are in a range that reflects a compromise between weight retention and optical beauty—somewhere between 50 and 66 percent. This may seem like a wide range, but in a 1-ct. stone, it’s a difference of less than one millimeter.