IMPORTANT: You must SELECT a stone from the Select Gemstone list box. If you type in Corundum (for example) you will get inaccurate results. Select the stone variety by typing the first two or three letters of your target stone. If necessary, arrow down until the correct gem is highlighted and press Enter.

Overview:

Cabochon weight is determined by many factors but this system concentrates on the visual bulge of the stone. The other factors are taken care of by the measurement data you supply for length, width and depth as well as the shape and species of the stone. The bulge of cabs that have the same basic measurements can vary widely. For example, you have two cabs that both measure 18x13x6. In our system we describe this as a Very High Dome. In the first example the sides of the dome are sloped in an arc that is very similar to this:

In the second example the stone has sides that are much sharper, straight up the sides, resembling this:

Both stones measure 18mm x 13mm x 6mm yet (assuming both stones are Agate) the first weight estimate is 8.79 carats while the second weight estimate is 11.36 carats. This is a difference of over 29% which is far too inaccurate of an estimate. This system was designed to overcome these obstacles in cabochon weight estimation.
 
 Using the Cabochon Weight Estimator:

 
This module is laid out in four sections (above you can see four boxes, each with a red number). You need to follow the four boxes in order. This is especially important for boxes 2 and 3. If you make an adjustment in box 3 and then go back and make an adjustment in box 2 you will have to repeat box 3.

Box 1 (Data): In the first section you enter your stone data. Click on the Select Gemstone dialog box. Select the stone variety by typing the first two or three letters of your target stone. If necessary, arrow down until the correct gem is highlighted and press Enter. Length, width and depth are to be entered in millimeters. Corrections: Corrections is a data field for making adjustments for odd shapes or extreme bulge not accounted for in the profiles. Say you had an oval stone but the shoulders were squared off instead of being rounded. This might require a Correction of +10%. In the Correction field you would type 10 (whole number 10 not .10). Do not add a + sign and do not add the % sign. If you wanted to subtract a percentage (say 5%) you would type -5 into the Correction field. If you wanted to add 2.5% you would type 2.5 into the Corrections field.

Box 2 (End View): While holding the stone in tweezers view the stone profile from the End. Use the + or - buttons with the mouse to increase or decrease the amount of bulge you see. Try to match the bulge of the stone with the bulge of the end profile. Keep in mind that you will rarely see a bulge outline that exactly matches the stone. Just get as close as you can. It is best to take the profile image (by pressing the + or - buttons with the mouse) past the actual bulge of the stone in both directions. This is the easiest way to ensure you're as close to the bulge as you can get.

You may have noticed that anytime you change the End Profile the Side Profile (in box #3) changes with it. This is because that most of the time the two profiles will be very similar. Anytime you change Box #2 Box #3 will change with it.

Box 3 (Side View): Now do the same with the Side Profile of the cab. Adjust the + or - buttons until the outline matches the stone as closely as possible. Always do this adjustment after you’ve adjusted Box #2 because if you go back and adjust Box #2 it automatically changes Box #3 to match it.

Box 4 (Weigh): Hit the Weigh button. In box 4 you will also see the Specific Gravity range (normal, high SG and low SG). At the bottom of the screen after you've hit the Weigh button you will see the Normal weight estimation along with the possible high SG weight and low SG weight. This simply means that if your gemstone had the highest SG found for that species then it could weigh as much as the highest weight shown if all the measurements are accurate and if the bulge of the stone matches your estimate.

Tip: Round Stones -  If you are working on a round stone look carefully at the profile while in the End View box. If the profile is an excellent match then you won't need to adjust box 4. However, you can fine tune your estimate if the End profile is a "tweener." That is if the profile is in between two bulge profiles, say, + and ++ (as an example), select + in box 3 (End View). Then in box 4 (Side View) select ++. This will split the difference between the two profiles, making your estimate more accurate.

Tip: Poor Match on Side View - If you are working with a stone that is very narrow you may end up with a situation where the Side View isn't even close. In this case it works best if you adjust the End View and don't adjust the Side View.

Tip: Re-Adjusting Box 2 - Remember that if you go back and re-adjust Box 2 (after adjusting Box 3) you will have to re-adjust Box 3 again. The reason for this is that the profiles of both boxes are always figured into the equation. If you are working on a Round stone you typically don't need to adjust Box 3 but if it didn't automatically change then the equation would be off.

Why doesn't the weight change when using the Profile Bottom of Stone (bottom bulge)?  I came up with a very surprising result when testing the stones I had with bottom bulge. Here's what I found: I measured the stones with bulge, weighed them and tried to figure out how much I had to add for the bulge. Incredibly, the first estimates were pretty close to "on the money." I didn't believe it at first because EVERYONE (GIA, Carmona etc) has said you need to ADD weight if there is bottom bulge. My intention was to remove the bottom bulge option and let everyone add the weight in the Corrections field.

A question arose: How could there be NO increase on a stone with bottom bulge? A theory: Take an oval Ruby with a high dome and a moderate bottom bulge that measures 8x6x5. Cut the stone into two cabochons (we're using a Super Laser here so there is NO weight loss!). The cut is made exactly where the bottom bulge begins so that we have two flat bottomed cabs. The two stones we end up with after our Super Laser cut are 8x6x3.5 (the original top) and 8x6x1.5 (the original bottom) so the 3.5 height and 1.5 height equal the original 5mm height. Run the real 8x6x5 ruby through the Cabochon Weight Estimator with a flat bottom and you get 2.50 carats. Run the 8x6x3.5 and you get 1.75 carats. Finally, run the 8x6x1.5 bottom and you get .75 carats. Add those two together and you get the same 2.50 carats. Kind of amazing, eh? Not all calculations are going to be that clean but if you change the top to 8x6x4 and the bottom to 8x6x1 you get the same result. If you don't believe this is accurate you can add any percentage you want into the Corrections field to compensate as you see fit.

Note: Small stones are the least accurate in terms of percentage.

GIA has stated that weight estimations should be 10% or less in accuracy to be of use. Testing of this system has shown the majority of estimates to be within 2.5% and the vast majority of estimates to be within 5% or less.