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“Could This Gem be a Little Bigger?” (How Big is Your Wallet?)

Posted by Yvonne Raley on

I get asked the question posed in the title about once a week.  Two out of three times I don’t hear back after my response.  I guess it's time to cut down on some of these questions, and put the request for bigger gems into perspective.
Let’s start with basic gem sizes and weights: each mm increase in diameter roughly doubles the weight of the gem.  Since gem prices are determined by weight, this also doubles the cost (or more, but I’ll get to that later).
An approximation of colored stone weight can be given by referring to standard diamond weights.  Here’s the standard table for brilliant cut diamond rounds:
1.3 mm
.01 carat
2.1 mm
.04 carat
3 mm
.1 carat
3.4 mm
.15 carat
4 mm
.25 carat
5 mm
.50 carat
6 mm
.85 carat
Now let’s make some sample adjustments for other cuts:
4mm round = 5x3 oval (1mm more in length, 1mm less in width, now you can do the rest)
5mm square = about 1.5 times the weight of a 5mm round (because you have to add in the corners)
5mm triangle = about 1/3 less than a 5mm round (because you cut down from the diameter)
Note that cabochons weigh roughly twice the amount of the same size faceted gem. 
Next, there are some basic adjustments for colored stones:
1. Most colored stones are denser, so they weight a little more.
2. Most colored stones are cut more deeply for color, so they weigh more again (if they are brilliant cut, they are cut like diamonds, but step cut for instance is rounder in the back and therefore heavier).
A good rule of thumb is that colored weigh on average 1/3 more than diamonds of the same weight.  So a 6mm round, or a 7x5mm oval weighs about 1 carat or a little more.
Finally, with some gems the size affects per carat price – drastically.  So let’s look into that.
First, we can exclude some gemstones where the per carat prices stay relatively constant, regardless of size.  That’s Amethyst, Apatite, Citrine, Chrome Diopside, Iolite, Mexican fire opal, Moonstone (and Labradorite), Quartz (any not already listed), Spessartite, Topaz (any)
Zircon.  Take this list with a grain of salt; there are always exceptions for special cuts or huge sizes.
Next, here are some gemstones where the per carat price jumps only in the large sizes.  The equal sign denotes price constancy.  The multipliers are in terms of the base price.  Also, I am only here talking about untreated stones.  So no heat, no oil, no glass filling   Oiled emeralds are probably similar in price, heated sapphires might be up to half these prices, but glass filled rubies are essentially worthless.
(Disclaimer: these multipliers are rough guidelines at best.)

Below 1 ct
1-2 ct
3-5 ct
5+ ct
Aquamarine
=
=
x 4
x 10
Peridot
=
x 2
x 2
x 10 minimum 
Red Garnet
=
=
=
rare
Rhodolite Garnet
=
=
x 3
x 5- x 10
Spinel
=
x 5 or more
rare
NA
Tourmaline
=
x 2
x 10
x 10
Tsavorite Garnet
x 5 after .25 carat
x 10
rare
NA

There are many exceptions to this, i.e. huge sizes in aqua or peridot can command prices in the hundreds of thousands.  Origin also plays a role with most of these gems.  Other prices fluctuate according to demand. 
Lastly, here are the gemstones where the per carat price jumps drastically starting with small sizes:

.10-.25ct
.25-.5ct
.5-1ct
1ct
2ct
3ct
4-5ct
Alexandrite
x 2
x 4
x 10+
x 20+
Rare
NA
NA
Diamond
=
x 2
x 4
*
*
*
*
Emerald
=
x 3
x 6
x 10
x 15
*
*
Ruby (Burma)
x 2
x 5
x 20
x 45
x 100
x 1000
x 1000
Sapphire (Ceylon)
x 2
x 4
x 12
x 30
x 30
x 30
*
Sapphire (Australian, Thai)
=
=
x 4
x 8
x 10
x 15
*

* These are prices I was asked not to provide.  Not so much that they’re a secret, but the fluctuations can be drastic.  Suffice it to say that a large diamond approximates the price of a large ruby, or more.  Diamonds are more available but controlled, rubies (Burmese, untreated) in the 3+ carat size are nearly non-existent.   So those two are more or less the two most expensive gems you can buy in large sizes.  Emeralds are cheaper and larger sizes are available, but they too can be in the hundreds of thousands in the very large sizes (5 carats or more).  Sapphires are also more abundant and will cost quite a bit less than rubies in the large sizes.
To those of you who have been quoted prices by me that are blow this list: congratulations. I probably got a good deal and passed it on. 
More about how to beat prices on gems in my next entry…

Matched Pair of 4mm Unheated Ceylon Sapphires I recently acquired


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