Fluorescence

Fluorescence is a type of color reaction to Ultraviolet radiation.
Fluorescence is seen under Ultraviolet light  in colors such as blue, white, and in some cases yellow, red, orange, and green, with an intensity of very slight, faint, medium, strong, and very strong.
To understand better what is fluorescence (without touching the chemical part) imagine taking a cup of water and pouring some milk in to it, that milky color is what fluorescence looks like.
UV light is everywhere, the sun produces UV radiation which most of it gets blocked by the Ozone Layer, but UV light is presence everywhere.
Fluorescence has nothing to do with clarity or color, it is possible to see a D color, FL clarity with very strong fluorescence.
Does fluorescence affect the value?
Depending on the appraiser that you ask, some will say yes and others no.
Investment Diamonds shouldn't have fluorescence, for the simple reason that some appraiser say it has an effect, always remember Investment Diamonds have to be bulletproof to all bullets, not just to some, sometimes the words of one appraiser can destroy your investment, so why give him the opportunity.

Weight estimation formula for round Diamonds

Once the Average Girdle Diameter and the Depth are discovered it is possible estimate the weight of a Diamond just by applying a simple formula.
The formula is as follows;
Average Girdle Diameter x Average Girdle Diameter x Depth x 0.0061
So a Diamond with a Average Girdle Diameter of 6.5mm and Depth of 4mm will wight:
6.5 x 6.5 x 4 x 0.0061 = 1.03ct
It is important to know that the formula works only for Diamonds with good proportions, in some cases some modifications have to be made, so never rely only on the formula (non of the changes is extremely big, however if the estimated weight is 1.00ct it is possible that the Diamonds real wight is 0.94ct which have a lower per-carat price).

Specific Gravity

Diamonds like every other mineral have a Specific Gravity, Specific Gravity (in the Diamond industry) is the ratio of the Diamond weight to the weight of an equal volume of water, in other words the ratio of the density of a Diamond to the density of water.
The rule is that higher the Specific Gravity the heavier a mineral is from an equal volume of a different mineral with a lower Specific Gravity.
 For examplae:
The Specific Gravity of a Diamond is 3.52, the Specific Gravity of a Cubic Zirconia is 5.80 (approximately), so a Cubic Zirconia of the same size as a natural Diamond will weigh more.

The Table

The Table has an extremely important role in the brightness and fire of a Diamond.
It is the biggest facet and largest visible part of the Diamond with an unaided eye, the table is essentially a door that lets light enter end exit the Diamond.
The Table size is generally expressed by its relation to the Diamonds diameter, some times it is expressed as "Table Size" and in most cases as "Table Percentage".
For example if the Diamond diameter is 100mm and the Table is 56mm, the "Table size" will be 56mm and the "Table Percentage" will be 56%, if the Diamond diameter is 8.05mm and the Table is 4.66mm, the Table size will be 4.66mm and the "Table Percentage" will be 57.88% [the equation is ("table size" x 100) divided by the Diamond diameter).
Most modern cut diamonds have a Table Percentage starting at 55% to 65%, Some old cuts have 45% or lower.

Total Depth Percentage

Once you know the Average Girdle Diameter and the Depth, the next step in order to get the Total Depth Percentage would be as follow:
Depth divided by the Average Girdle Diameter X 100 = Total Depth Percentage.
For Example:
Depth = 3.50
Average Girdle Diameter = 6.20
(3.50 / 6.20) X 100 = Total Depth Percentage.
(0.564) X 100 = Total Depth Percentage.
56.45%  = Total Depth Percentage.
For Jewelry the ideal Total Depth Percentage is somewhere between 55% - 65%.
However when it comes to investment the ideal Total Depth Percentage would be 58% - 62%.
It is important to add the even if the Diamond is within the safe Total Depth Percentage there is no guarantee that its a excellent cut, there are many other factors involve, and it only takes one to jeopardize the investment.

Depth.

Depth is the second step needed to obtain the Total depth percentage, depth is the distance (in millimeters) from the top of the Diamond (the table) to the bottom (the culet).

Average Girdle Diameter.

Average girdle diameter is the first and most basic measurement taken, as you know every proportion of a Diamond is essential to its interaction with light, a shining Face-up look is extremely important in a Diamond.
The average girdle diameter is calculated by measuring the diameter of a round diamond from several locations in the girdle, adding the smallest and largest and dividing by 2. (all the measurements in a Diamond are expressed in millimeters)
For example:
after taking 4 measurements the biggest was 2.30mm and the smallest 2.22mm.
2.30mm + 2.22mm = 4.52mm
4.52mm / 2mm = 2.26mm 
The Average girdle diameter is 2.26 mm.
Average girdle diameter is only one of two steps needed to get the Total depth percentage which is the most important factor in the Diamond cut.