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Operating Diamond Tester


Just got a diamond tester and I have a couple of questions:

  1. it mentions adjusting/calibrating for different sized stones…but
    it doesn’t tell me how. Do you adjust the sensitivity up or down for
    small stones?

  2. Why will two stones of similar size react differently: f. ex. one
    will zoom up immediately to the diamond level, while another will
    slowly climb up and then beep.

  3. Do inclusions affect the accuracy of the readings? Am thinking
    larger carbon inclusions, for ex.

  4. the tester has a UV light on it. Do ALL diamonds fluoresce or
    just some?

Jeannius Designs


Hi Jeanne,

I can’t answer your questions about the diamond tester for the same
reason you need to ask the questions at all; the darn things baffle
me. I’ve tried them with very similar results to yours. However, a
good examination with a microscope, showing characteristic
inclusions, razor-sharp facet junctions, as well as other tests,
including UV fluorescence are usually sufficient to identify and
grade them. Even practicing with a 10X loupe can be a more accurate
way to separate diamonds from their imitations and substitutes…but
it does take practice.

I can answer the last question, though. Some diamonds fluoresce,
some don’t. Most of the ones that do, fluoresce a faint to strong
blue. Sometimes, certain diamonds that look “hazy” or cloudy under
magnification will fluoresce a weird greenish-Yellow. After you’ve
seen a few of these, you’ll come to expect the phenomenon. The
fluorescence is caused by the same thing that colors most gemstones;
intervalence charge transfers between neighboring ions (free

I don’t know how powerful your tester’s UV lamp is, but try to use
it in the darkest environment possible. I carry a small keychain type
at all times, but it is a poor substitute for a full-on
longwave/shortwave UV lightbox. I realize that gemologiccal equipment
is expensive and learning how to use it can be more so, but not as
expensive as buying what you thought were diamonds because your
uncalibrated (or just plain unreliable) diamond tester told you they

Please don’t take my last comment as criticism, I’m only relating my
own experiences and would love nothing more than to provide a
reliable electronic/mechanical solution for your diamond tester. Its
just that the reality is no battery-operated machine can decide as
well as a trained eye combined with more reliable equipment.

Best, Jeanne,
James S. Duncan, G.G.
James in SoFL


Oops, I forgot to answer question #3

Do inclusions affect the accuracy of the readings? Am thinking
larger carbon inclusions, for ex. 

Only if the inclusion reaches the surface and you touch the tester’s
tip to the included crystal it might affect the reading, but it is
unlikely to occur. Most surface-reaching crystal inclusions tend to
pop out of diamonds during polishing, leaving a cavity. If it
remains, it is called a “knot.”

Thermal diamond testers have a metal tip that heats up, then
measures how quickly the material conducts heat away from the tip. If
the interval is that of the thermal conductivity of diamond, it
registers that. If not, it registers as a simulant. Testers also have
a recovery, or “cool down” period between taking readings. If the tip
isn’t allowed to recover, it will give different readings, which may
be a partial answer to question #2.

James S. Duncan, G.G.
James in SoFl


I just got a Presidium DiamondMate. The book says it can test for
sapphires, but it doesn’t say what happens when its a sapphire. Does
anyone know? Can you give me some insight on my new toy?


The Presidium support center is in KY. Their number is 877-514-3693,
or 859-514-3694. Hope they can answer your questions.

The Jewelry Equipment Dr.