Ass # 12 - Grading Table and Proportions

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Questions and Answers

What is the first step in determining the average girdle diameter of a round brilliant diamond?

Calculate the total depth percentage to assess the weight.

Measure the depth from table to culet.

Determine the table percentage based on the girdle diameter.

Add the smallest and largest diameter measurements and divide by two. (correct)

What indicates a diamond is likely underweight based on total depth percentage?

Total depth percentage of 70.0 percent or more.

Total depth percentage between 60.0 and 65.0 percent.

Average girdle diameter measurement below 5 mm.

Total depth percentage below 55.0 percent. (correct)

If measuring the total depth of a diamond to calculate total depth percentage, how should the final result be reported?

As a fraction of the average girdle diameter.

Rounded to the nearest tenth of a percent. (correct)

Rounded to the nearest whole number.

As an exact whole number.

Which range of table percentages is most common for round brilliant diamonds?

55 percent to 65 percent. Signup and view all the answers

What measurement tool is necessary to measure a diamond’s table accurately?

A table gauge. Signup and view all the answers

What percentage indicates that the table bows out noticeably?

67 percent Signup and view all the answers

Which crown angle range is generally considered attractive for diamonds?

25 to 35 degrees Signup and view all the answers

How should table percentages be adjusted based on star length?

Add or subtract depending on the distance from table edge to star Signup and view all the answers

Which method is used to estimate the crown angle by observing the diamond under magnification?

Profile Method Signup and view all the answers

What is the common range for star length percentages?

50 to 55 percent Signup and view all the answers

Which crown proportion is generally deemed more significant in the diamond trade?

Crown angle Signup and view all the answers

What happens to the appearance of a diamond if the crown angle is below 25 degrees?

Generally looks unattractive Signup and view all the answers

In the Face Up Method, what does the comparison of widths help determine?

The steepness of the crown angle Signup and view all the answers

What is one of the primary functions of the girdle in a diamond?

To prevent damage Signup and view all the answers

Which architectural feature of a diamond is typically assessed at the valley positions of the girdle?

Girdle thickness Signup and view all the answers

What impact does a thick unpolished girdle have on a diamond's appearance?

It causes a gray reflection Signup and view all the answers

How is girdle thickness usually reported for diamonds?

In a thin-to-thick range Signup and view all the answers

What technique do cutters sometimes use to alter the angles of the diamond to conserve weight?

Digging out Signup and view all the answers

What is the purpose of measuring the diamond table from corner to opposite corner four times?

To determine the average table diameter. Signup and view all the answers

What is the effect of a shallow crown angle on the characteristics of a diamond?

It typically results in large tables and thin girdles Signup and view all the answers

What is the primary factor used to determine the crown height percentage of a diamond?

Crown angle and table percentage Signup and view all the answers

What does the term 'table percentage' specifically refer to?

The relationship between the average table diameter and average girdle diameter. Signup and view all the answers

Which method might a polisher use to create thinner girdle positions on a diamond?

Digging out Signup and view all the answers

When using the bowing method to evaluate a diamond, what should you look for?

The straightness or curvature of lines at the star facets. Signup and view all the answers

Which flash size indicates that the diamond's table size is likely less than 60 percent?

Small flash Signup and view all the answers

Why is it important to inspect the entire circumference of a diamond's girdle?

To assess girdle thickness accurately Signup and view all the answers

What is the consequence of using excessive thickness in a diamond girdle?

It may add unnecessary weight without aesthetic benefit Signup and view all the answers

In the ratio method, what does a 1:1 ratio indicate about the diamond?

The table diameter is exactly the same as the distance to the culet. Signup and view all the answers

What contributes to an accurate estimate of table size using the flash method?

The light reflecting off the table. Signup and view all the answers

Which of these methods would you use if you are looking at the diamond under magnification with darkfield lighting?

Ratio method. Signup and view all the answers

To achieve an accurate Diamonds Table Percentage, which values are needed for the calculation?

Average table diameter and average girdle diameter. Signup and view all the answers

What does it imply if the table size is evaluated to be in the range of 65 percent or greater using the flash method?

The flash appears large compared to the face-up view. Signup and view all the answers

Which of the following statements is true regarding the function of the girdle?

The girdle protects the diamond's facets from wear. Signup and view all the answers

How many crown angles on a round brilliant diamond should be estimated before averaging?

8 Signup and view all the answers

Study Notes

Diamond Grading Proportions - Table, Crown, Girdle

Average Girdle Diameter: Calculated by averaging the smallest and largest girdle diameter measurements of a round brilliant.
Total Depth Percentage: Represents the diamond's table-to-culet depth as a percentage of its average girdle diameter. Measure from the table to the culet with a millimeter gauge.
Underweight Diamonds: Have total depth percentages below 55.0%, often characterized by shallow crowns and/or pavilions.
Overweight Diamonds: Have total depth percentages of 65.0% or more, often characterized by steep crowns and/or deep pavilions.
Table Percentage: The table size expressed as a percentage of the average girdle diameter. Most round brilliants have table percentages between 55 and 65 percent. Fancy shapes might have slightly smaller or larger tables.

Direct Table Measurement

Measuring the Table: Use a transparent millimeter gauge to measure the table facet from corner to corner four times.
Average Table Diameter: Calculating the average table diameter: (sum of four table measurements) ÷ 4
Table Percentage Calculation: (Average Table Diameter ÷ Average Girdle Diameter) × 100

Estimating Table Percentage

Flash Method: The size of the light reflected off the table gives a visual estimate of the percentage. Small flash = under 60%. Medium flash = ~60-64%. Large flash = over 65%.
Ratio Method: Compare the culet and outer parts of the diamond measured under magnification, both should have the same length for a 1:1 ratio.
Bowing Method: Inspect the straightness of lines between star facets on the table. Straight lines = ~60%, Slightly bowing in = ~58%, Slight bowing out = ~63%, Significant bowing out = ~67%.

The Crown

Crown Angle: Angle formed by the girdle plane and the bezel facets.
Crown Height Percentage: Distance from the girdle plane to the table expressed as a percentage of the average girdle diameter. Crown angles from 25 to 35 degrees are generally preferable.

The Girdle

Girdle Function: Prevents damage and provides a setting edge. Thick girdles add weight without much visual impact.
Girdle Thickness Assessment: Evaluate the thickness of the girdle's "valleys and hills" under a 10x magnification to assess the thickness.
Painted/Digging out of Facets: Techniques used to selectively remove or adjust facet thickness. The process of painting allows light to be gently polished to remove less thickness from certain areas whereas digging out allows facets to be tilted towards one another or away to change areas of thickness.

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Description

This quiz covers important aspects of diamond grading, focusing on the proportions of the diamond's girdle, total depth percent, and table measurement techniques. Understand how to distinguish between underweight and overweight diamonds based on these measurements. Test your knowledge on how these factors impact the overall evaluation of diamonds.