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Ruby Vs. Garnet

Hi all,

How could you tell if a stone was a ruby or a garnet. I have a few
rough stones and we believe they are ruby’s, but how can I be sure.

Thanks all in advance.


What you have may be a garnet, ruby, tourmaline, spinel, red beryl
or other red stone. It’s not hard to separate garnet from ruby , but
without some gem testing instruments and the knowledge of how to use
them there is really no way for you to tell. One instrument you
could use is a polariscope. You can jury rig one out of a couple of
polaroid lenses from a pair of sunglasses. Having almost unlimited
faith in the powers of Google, I suspect if you were to google the
word" polariscope" you could come up with a pretty good description
of how to make and use one. Watch out though! This gemology stuff
can be addicting to some people! If you don’t want to there may be a
rock club in your area, you might contact someone from the club and
ask if there is someone they know who can help. The easiest way of
course would be to take them to a gemologist.

Jerry in Kodiak

Get some polarized glasses and cut the lenses in half, orient the
filter, you can also buy polarized filters at any pawn shop camera
section. Then place the filters so one is 180 degrees opposed to the
other. As You turn two filters they will cut the light, at that
point they are 180 degrees opposed then mark and cut, place this in
between your stone and a light source, garnets will show red and
yellow at that point rubies do not.


One way is if it has any crystal structure to it…ie. Natural
facets. A ruby has a hexagonal/cylindrical structure and garnets are
usually more round w/ facets (don’t remember exactly how many sides!
Sorry)…otherwise, ruby is harder than garnet, though not by that
much. The most certain way to find out is to use a refractometer to
read the angle at which light bends on entry to the stone. This is
individual for the different gem types. If you don’t have one, ask
your local gemologist.



Do you have Gem testing equipment? If so, first start off with a
Polariscope to determine if the stone is a single or doubly
refractive material ( Single refraction : Garnet & Double
Refraction: Corundum)

Next use a Refractometer to determine the Refractive Index ( Garnet:
from a low of approximately 1.72 up to a high of approximately 1.875
depending on the type and Corundum: 1.762 - 1.770 ( -.003,+.008) ).

Next you can do a specific gravity test ( Garnet range: 3.61 to 4.15
depending on type & Corundum: 4.00 (+ or - .03) )

And finally you can examine the stone for inclusions but that is too
detailed for me to explain here. Pick up a good book that has photos
of the inclusions or a good explanation such as The Handbook of Gem
Identification by Richard T. Liddicoat, Jr.

Handbook of gem identification
By Richard T Liddicoat

If you do not have experience as a Gemologist it may be best to
consult one.

Good Luck
Greg DeMark
Custom Jewelry - Handmade Jewelry - Antique Jewelry

How could you tell if a stone was a ruby or a garnet. I have a few
rough stones and we believe they are ruby's, but how can I be sure.

If you really want to be sure, have a gemologist test it. By the
general air of your post, I’ll assume you don’t have any gem testing
equipment. However, one test you may be able to do is with a
longwave ultraviolet light (the “black lights” used with posters).
If the stones in question are rubies, they may (or may not, but
probably will) glow bright red. Garnets will have no reaction. Keep
in mind that, even if the stones do have a reaction, they still may
not be ruby. Red spinel isn’t the only red gemstone apart from ruby
that has a UV reaction.

James in SoFl

Then place the filters so one is 180 degrees opposed to the other. 

I think that’s 90 degrees …



I believe your post could be confusing to the uninitiated concerning
polarized light. Let me try: When two polarized filters are oriented
properly, and arranged one in front of the other so that you look
through both of them , they can show the effects of polarized light
on an object that is located between the two filters.

How do you orient them properly? well, if they are from the same
stock, then hold one steady in one hand, in front of a light source.
Take the second filter, and hold it in line with the first so that
you look through them both. Rotate the second filter in the same
manner as a second hand on a clock rotates, while holding the first
one steady. You will see, at some point, the most amount of light
transmission, and then, approximately ninety degrees from that point
you will see the minimum amount of light transmission.

At this point of minimum transmission, use your third hand to place
the object to test between the filters. If it shifts the
polarization of the light, it will appear as a bright spot. Oh, it
would probably be best to use a non polarized light source for
viewing, such as regular incandescent light.

I have some scraps of this (polarizing film) material that i would
donate for experimentation if you contact me off-line.


oops…your right, hada brainfart… 90 degrees is correct. sorry
for the confusion