What is the units of measurement used for a gem’s weight? How are they related?
Carats (ct) and points (pt). 1 ct = 0.2 g; 1 pt = 0.01 ct
How can a 1.997 ct diamond be approximated?
It should be approximated as 1.99 ct. Usually, carats are approximated to the second decimal (ie X.XX); by trade carats are never rounded up unless the third decimal is 9 (ie 1.999 -> 2.00)
What is the definition of density? And the definition of heft?
The density is the ratio of a material weight over its volume and it’s usually espressed as g/cm3; heft describes the expectation of a heaviness in relation to its size. Heft is qualitative and does not measure the relative heaviness of a material.
What’s the definition of specific gravity (SG) ? What is its unit of measurement?
Specific gravity is defined as the ratio between the density of a material and the density of water. It can also be measured as the ration between the weights of the material and water, at room temperature. It has no unit of measurement.
Why is it useful to measure the specific gravity (SG) of a gem?
To distinguish a gem from others with similar aspect.
If a known gem cannot be taken out of its mount, how can its price be evaluated?
By careful measurements of its dimentions, the volume can be calculated; by knowing the gem, the specific gravity (SG) is known; multiplying the volume by SG and the density of water (1g/cm3), it is possible to evaluate the weight and so the price of the gem.
What are the most important aspects to consider while evaluating a mounted gem to avoid mistakes?
The measurement of the dimentions needs to be accurate;
Specifig gravity (SG) can be affected by inclusions and defects in the gem;
Proportion variations in the cut;
Mathematical errors.
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