FMF - Friends of Minerals Forum, discussion and message board

[João Luz]

So this was sold to me as a ruby, but I've now seen that corundum identification is not very linear. Can anyone help me identify this?

[ALshawani]

I do not think that sapphire, this stone is not transparent, and you have to measure its hardness and density to make sure of the type of stone

[Bob Morgan]

It definitely looks to be corundum with the diagonal lines on the prism faces, and red corundum is ruby.
A hardness test is to be desired. Also, the locality that it came from would help.

[Bob Carnein]

I agree with Bob. The diagonal cracks are most likely rhombohedral parting, which is typical of corundum. Whether you would call this ruby is another issue entirely.

[Tobi]

I think we can be sure that this is corundum, but João, what exactly is your question - if this crystal is corundum or if this corundum can be considered a ruby? ;-)

[João Luz]

Thank you everyone! :)

[Josele]

[Peter Lemkin]

I would almost be willing to bet it is from Tanzania - this location: https://www.mindat.org/loc-105821.html There are quite a lot of them now on the market and they can have a nice reddish color - some even when a bright light is shined through it, part will be translucent. I agree with the others, that technically 'ruby' is usually reserved for the clear, gem quality...but no one can stop you from putting 'corundum var. ruby' on your index card with the specimen. Mindat has several images from that location.

[Riccardo Modanesi]

Hi to everybody!
Ok, for calling it a ruby, it should contain at least some traces of chrome. I studied gemmology in Idar-Oberstein, as the biggest majority of you should know, and in the German Gemmological School they say "ohne Chrom kein Rubin" (i.e. no chrome, no ruby).
It should be defined what is the border between a "rough red corundum" and a "ruby". Many included red corundums are cut and marketed as "rubies", especially the Tanzanian and Mozambique ones.
Greetings from Italy by Riccardo.

[Peter Lemkin]

So, I think the consensus is: corundum with red color = yes ; ruby = no. It is sadly usual for some to sell specimens with a 'sexier' sounding name for commercial purposes.

[Amir Akhavan]

Again and again, the sensory thing in definitions...

When I studied biology, I took a course in psychophysics at the Max Planck Institute of Biocybernetics. The professor at one lab studied color perception. He was color blind, but he could accurately predict our visual perception, including color.
I can explain to a blind person what a smoky quartz is, and can you believe it, a smoky quartz is what it is even if I turn out the light.

To make transparency a criterium means the following: if you smash your translucent corundum, and pick up the small clear shards, they will be ruby. Likewise, if your ruby contains inclusions and grows too large, it turns into red corundum, what a pity.

Minerals are not defined by the sensory impression or qualities that influences their value as goods, they are defined by physical properties. That is a good thing, and the only way of dealing with them scientifically.
There is no reason to withhold what is good for dealing with them from their varieties. So if it is in any way possible, go by the physics. Ruby is corundum with an absorbtion spectrum caused by Cr impurities.

[James Catmur]

It always interests me (as a very color blind person) that we can learn how to tell colour from other visual things. People often used to ask me why I was so good at finding specimens and my answer was 'because I look for the things non-color blind people cannot see'. That said, I could never find malachite.

[João Luz]

Thank you all for the info!