Measuring the weight of diamonds and gemstones. The magic formulas....

Hi All,

I started this topic for all whom have shown an interest in estimating the weight of diamonds and gemstones.

In my next post under this topic I will be posting all of the informative text but much will be incomplete without some tables, and reference material. I haven't found an easy way to include those within this forum so I will ask all that are interested to PM me and I will forward them to you. Please include your email address as I cannot send it through the forum. Thx.

The information will include some helpful color gemstone indentification charts, diamond charts and the methods used to measure and estimate the carat weight of ALL gemstones with just a pair of calipers ($2 at Harbor Freight) and some simple math. This is the same method used by appraisers without the removal of the stones from the mountings.

Pawnbroker Bob



Edited 1 time(s). Last edit at 02/06/2012 10:28PM by Pawnbroker _Bob.

Am sure would be welcomed by many forum members.

My advice is know a respected diamond buyer but this info sure would give one an idea beforehand..

Understand clarity also comes into play and basically there are diamonds and there are diamonds and two the same size not always equal...

Dan-Pa., This fun topic is about measuring and calculating the WEIGHT of ANY gemstone or simulant in the mounting or loose. Very helpful if you don't have a carat scale. Us gemologists have reduced the tough geometry to a simple formula using a variable for the cut and the stones specific gravity to calculate the weight. It will also cover some basic colored stone identification. Not valuation.

If you are interested in the valuation and appraisal of stones and jewelry I encourage you to start another topic and I will help out.

/PB



Edited 1 time(s). Last edit at 02/07/2012 07:43PM by Pawnbroker _Bob.

Bob actually the nice stones I have found ended up on my wifes, daughters, inlaws, friend fingers so clarity or any data on diamonds means little to me as they usually stay away from the apprasiers but the last one the diamond guy said very pretty piece and thats enough for me...

Look forward to learning your stone measuring methods.

For those that have requested the materials... I will be sending them out this weekend with my instructional post. Sorry it is taking a little longer than expected. It has been a busy week.

/PB

Anticipation (sung as in the ketchup comercial)

HH
Mike

Sorry for the delay guys... my computer went on the fritz. I thought I posted everything last week but it appears I am still having a computer problem. My post has not shown here.

/PB

Hi Gents,

I apologize for the delay in this posting. My computer went on the fritz.

For those that requested the information you should have received four different documents.

To really do ourselves justice we should discuss gem identification, metal identification, and a bunch of other gemology topics that I spent years learning. But we have to start someplace and there has been a request about measuring diamonds and gemstones and weight. Because we are sort of putting the cart before the horse some terms may be unfamilier. Please ask if you need any clarification.

First off lets talk about calibrated gemstones. All diamonds up to about 18 pts and most colored gemstones used in commercial jewelry are all calibrated. What this means is that they are all cut to a specific size and mostly by machine. They are however sold by weight. It is no different than ordering a 2" thick cut rib eye steak from the butcher and then purchasing it by the pound. I order a Topaz the same way to fit in a mounting. You will notice on your CHEET SHEET that the sizes are all standardized mm x mm.

When we sort small diamonds we use sieves of different millimeters to sort them. The stones are then sorted to other specs and then sold or matched, weighed and mounted.

Almost all semi precious gemstones are calibrated. As the stones get larger, say 10 cts the more variation you will find.

Precious gems, ie.. diamonds rubies and emeralds, plus a few rare others over 20 points are cut to maximize weight (for greater value) and not considered to be calibrated. This where your math and formulas are going to be helpful!

BTW.. Marquise cut stones are considered the red headed stepchild. These are usually cut from rough that they can't figure out how to cut a nice stone from.

The math is easy, LENGTH X WIDTH X DEPTH X S.G. (Specific Gravity of the gem mineral) X (cut variable) = TOTAL WEIGHT. I have included the formulas used for most common cuts in your material. In addition to the most common gemstones , which I have color coded for help in identification, I have included a specifice gravity table of many other compounds that are used as simulants (fake) gems ie.. glass, CZ, YAG to name a few. Notice that CZ has a SG ~30% greater than diamond. NOTE: These formulas are calculated figuring an average cut stone. Variations can change this see below...

Specific gravity is a ratio of the density of a substance to an equal voulme of water. Water equals 1.0 so things that are lighter than waterand float will be less than 1.0 ie .82. Things that are heavier than water and sink are greater than 1.0. ie 1.3, 5.6. I have included a HUGE list of SG's for reference.

Measuring. For prong set stones it is easy. Where you run into difficulty is pave, gypsy or channel set stones. The answer is to GUESTIMATE. Yup, everyone does it even the high dollar appraisers. In an appraisal where the major stone is not taken from a mounting the appraiser notes that size is an estimate only, using standard measuring practices. Estimate where the edge (girdle) of the stone ends under the lip of metal and take your measurement there.

Measuring the depth can be challenging. We have an expensive tool called a Leverage Gauge that allows us to measure up through the cleaning holes in a piece of jewelry. If you can't get a direct measurement. ESTIMATE!. lol You wont be that far off after you have done this a few times. Find another unmounted stone of about the same dimensions and use it's depth. A tiny piece of rod (plastic, metal etc.) taped to the inside face of your caliper can also be used. One caliper face goes on the crown, measure the depth off the tip of the rod at the bottom, subtract the height of the rod... viola!

On round stones the formula indicates to square the diameter... I prefer to take two measurements at 90 degrees to each other and multiply. It is more accurate.

Cutting and variations. Most stones are cut to very exacting tolerances for the cut (style) of the stone. The facets and angles are calculated to maximize the capture of and dispersion of light. There is very little difference from substance to substance but there is some wiggle room. From the girdle of the stone up there is very little variance with respect to weight. Most weight variations will occur in the girdle or below with thin or THICK girdles, deep or shallow pavilions and straight or heavy shouldered pavilions. How do we deal with this? GUESTIMATE! If it is thick at the waist and a fat bottomed girl ADD 5-10-15% if it is skinny as a rail with a razor edge waist DEDUCT 5-10%.

Well that is how to estimate the weight of gemstones. Simple really.

Questions are welcomed.

/PB

Updated--- fixed my computer.

Hi Bob,
Thank you for the cheat sheets and the post. I appreciate all your effort.
Could we discuss gem indentification?

I have some stones that are red, some blue, some greatfruit pinkish, etc.

How would I positively identify my stones?

And how would I grade them?

Thanks again.
Mike