DDG Ass #12 Grading Proportions PDF

DDG Ass #12 Grading Proportion- Table, Crown, Girdle Average Girdle Diameter and Total Depth Percentage: - Average Girdle Diameter measurement is the basis for comparing the diamond’s other proportions, It's determined by adding the smallest and largest diameter measurements of a round brilliant a...

DDG Ass #12 Grading Proportion- Table, Crown, Girdle Average Girdle Diameter and Total Depth Percentage: - Average Girdle Diameter measurement is the basis for comparing the diamond’s other proportions, It's determined by adding the smallest and largest diameter measurements of a round brilliant and dividing by two. - Total Depth Percentage is the diamond’s table-to-culet depth expressed as a percentage of its average girdle diameter. -To determine a round brilliant’s average girdle diameter, measure it from one edge of the girdle directly across to the opposite edge. Diamonds are always measured in millimeters - Since no stone is perfectly round, take several diameter measurements. Average Girdle Diameter = (Minimum Girdle Diameter + Maximum Girdle Diameter) ÷ 2 - Once you know the diamond’s average girdle diameter, you can determine its total depth percentage. -Measure the diamond from table to culet with a millimeter gauge. Once you complete the equations round your result to the nearest tenth of a percent. Total Depth Percentage = (Depth ÷ Average Girdle Diameter) × 100 -Diamonds with total depth percentages below 55.0 percent are generally underweight, with shallow crowns, shallow pavilions, or a combination of both. -Diamonds with total depth percentages of 65.0 percent or more are generally overweight, with steep crowns, deep pavilions, or both. - while a total depth percentage of 60.0 percent indicates good proportions in a diamond, it doesn’t guarantee them cause it might have negatives like a shallow crown, a very thick girdle, or a deep pavilion. Table and Star Facets: -The table is the largest facet on a diamond. -Table size is stated as table percentage: a percentage of the stone’s average girdle diameter. - Most cutters produce round brilliants with table percentages that range from about 55 percent to about 65 percent. -Tables on fancy shapes might be slightly smaller or larger. Direct Table Measurement: -To measure a diamond’s table, you need a table gauge, a transparent measuring device divided into millimeters and tenths of a millimeter (0.1 mm). -Measure the table facet from corner to opposite corner 4 times -To get a Diamonds Table Percentage you first need to find the average table diameter by using this equation: Average Table Diameter = (Table Measurement + Table Measurement + Table Measurement + Table Measurement) ÷ 4 -Now take this number along with average girdle diameter to get table percentage Table Percentage = (Average Table Diameter ÷ Average Girdle Diameter) × 100 Estimating Table Percentage: -With practice, table size estimates can be fairly accurate, especially with diamonds that have table percentages between 50 and 70 percent. -Flash Method: - Using the flash of white you see is the light reflecting off the table. Your perception of the size of this flash determines your estimate of the table percentage. - If the flash is small compared to the entire face-up view of the stone, the table size is less than 60 percent. -A medium-sized flash indicates the table size is somewhere between 60 and 64 percent. - If the flash is large, the table size is probably 65 percent or greater. Ratio Method: - To use the ratio method, look at the stone in the face-up position under magnification with darkfield lighting and you should see the culet centered under the table. -Compare the outer section to the inner section, which runs from table edge to culet. When both sections are the same length, the ratio is one to one (1:1). As the table gets larger, the inner distance increases. Bowing Method: - Look at the diamond face up, identify the line that begins at the point of one star facet and travels along the table edge to the point of the next star facet. Four of these lines form the outline of a box. - Evaluate one line at a time. Judge whether each line is straight or whether it bows, or curves, in or out. Use the percentages below to assign a value to each line: If they bow in noticeably, table percentage is about 53 percent If they bow in slightly, table percentage is about 58 percent If they don’t bow at all—if the lines are straight—table percentage is about 60 percent If they bow out slightly, table percentage is about 63 percent If they bow out noticeably, table percentage is about 67 percent -Star facet length—the distance between the base of the star and the point—might affect your estimate of the diamond’s table size - Depending on the distance on this distance you will add or subtract to the bow percentage you determined Star Length Percentage: - star length percentage is a percentage of the total distance between girdle edge and table edge. -Examine all eight star facets as you work your way clockwise around the gem. Once you have estimates for all eight star facets, average them and round that figure to the nearest 5 percent. - Star length percentages typically range from 50 to 55 percent. Outside that range, long stars (65 to 70 percent) are more common than short stars. Very short stars below 35 percent are rare. The Crown: - The two crown proportions that can affect a diamond’s appearance are crown angle and crown height percentage. -Crown angle is the angle formed by the bezel facets and the girdle plane. -Crown height percentage is the distance from the girdle plane to the table, expressed as a percentage of average girdle diameter. -In the trade, crown angle is considered more important than crown height. - crown angles from about 25 to about 35 degrees produce attractive, bright, and fiery diamonds if other proportions are compatible. Estimating Crown Angle: Profile Method -To estimate the crown angle using the profile method, hold the diamond table-to-culet and look at it under magnification with overhead or darkfield light. -. Make sure you’re looking at bezel facets and not the star and upper half facets. Face Up Method: -Mentally divide the pavilion main into two parts: one that runs from the culet to the corner where the table meets the bezel facet, and one that runs through the bezel facet to the girdle edge. -Compare its width where it meets the table corner to its width at the top point of the bezel facet. -The greater the difference between the two widths, the steeper the crown angle. The Crown - Rough -Stones cut from shallow rough often have shallow crown angles because the cutters tried to get the most weight along with the largest girdle diameter and greatest face-up size which can negatively effect other proportions -As a result, stones with shallow crown angles also tend to have large tables, thin girdles, and shallow pavilions. - To determine the crown height percentage, you have to know the diamond’s table percentage and crown angle. Once you’ve determined these figures, refer to the Crown Height Percentage Chart. The Girdle: -The girdle's functions are to prevent damage and provide an edge for setting the stone into jewelry. -The girdle should be thick enough to prevent chipping. Extra thickness adds weight, but contributes nothing to a stone’s appearance. - A thick, unpolished girdle can detract from the diamond’s appearance by causing a gray reflection in the stone. -On a standard round brilliant, the girdle consists of 16 narrow “valley” and 16 broader “hill” positions. Assess girdle thickness at the valley positions, where the upper and lower half facets are closest to each other. Estimating the Girdle Thickeness -To assess girdle thickness, examine the entire circumference of the stone, in profile, under 10x magnification with darkfield lighting. -Girdle thickness is usually reported in a thin-to-thick range—for example, Very Thin to Medium -An extra facet, natural, chip, cavity, or indented natural located on the crown or pavilion at the girdle edge might narrow the girdle. When this happens, consider only the remaining girdle areas in your thickness assessment. Thickness Variations: -a normal girdle, the hill positions are all about the same thickness, resulting in fairly even scalloping all around the diamond. - the polisher sometimes uses weight-saving techniques called painting and digging out to change the angles of the upper and lower half facets in relation to the horizontal plane -Painting is a process of very light polishing that allows the cutter to gently polish away less thickness at selected locations around the girdle. - Digging Out the cutter tilts the upper or lower half facets, or both, away from the bezel or pavilion facets and toward each other. The hill positions where the upper half facet junctions meet the lower half facet junctions are thinner than those where the bezel facets meet the pavilion main facets. - Cutters often use digging out to remove naturals or blemishes at the girdle. -Digging out might produce dark upper half facets and make the stone look dark face-up. It also makes the gem look smaller because the diamond’s brightness doesn’t extend to the girdle edge. -Severe digging out might make some crown facet junctions appear less sharp, to the point where adjacent upper half facets look like a single large facet. -Severe painting and digging out can result in significant thickness differences between adjacent hill positions and cause uneven scalloping around the diamond’s girdle