Measuring Table Size
The most accurate way to measure a table is with a table gauge. This is a small, transparent, plastic ruler that measures in increments of tenths of a millimeter (0.1 mm). A scope or a loupe is required to measure the table in four directions with your table gauge. Measure the distance across the table starting between two stars (where the points join) to exact opposite location. Repeat this three more times for the other possible directions across the table, recording each measurement. Try to record these measurements in hundredths of a millimeter for accuracy. For example, if the table measures almost half way between tenths of a millimeter record this as 3.54 mm. Half way between the tenths would be recorded as 3.55 and so on. The largest measurement of the four will be used for the final size.
At first it will probably be difficult to measure a diamond's table while trying to keep the gauge absolutely steady and the loupe in perfect focus. When using a loupe, place the pavilion into a slotted support of some type, such as a piece of cardboard fashioned to keep the diamond upright. Steady your hand by resting it on a surface while taking your measurements.
The second part of the table percentage value is the girdle diameter. This is much easier to measure with an A.D. Leveridge Gauge. Place the table side down on the gauge platform between the flat jaws, then rotate the diamond, making a note of the smallest and largest measurement values. If the diamond is mounted, use the outside needle-jaws to fit in between prongs. Add these two figures together and divide by two getting to get the average girdle diameter. Finally, to find the table percentage, divide the largest table measurement by the average girdle diameter.
Girdle Diameters: largest: 6.32 smallest: 6.24, [6.32 + 6.24 = 12.56] x .5 = 6.28 (average girdle diameter)
Table Diameters, 4 measurements; 3.43, 3.46, 3.45, 3.44;
Largest Table Diameter, 3.46. Divide by average girdle diameter (6.28) = .55095
Round this figure to the closest two-digit number and express in %: 55%.