FMF - Minerals Forum, discussion and message board

[Eduardoo]

Hola amigos:

I found this crystal, it weights 308c. Photos do not make it justice as the transparency does not show.

Since I am new at this I don´t know if it is an important piece, but I am excited about it.

Any comments will be appreciated.

Regards, Eduardo.

[John S. White]

Looks like a water worn topaz to me. Even if it were a beryl, it does not make it as an aquamarine, which is greenish blue to blue in color.

[Eduardoo]

Really the piece looks better than the photos show.

Many thanks,

Eduardo.

[GneissWare]

I agree with John--it looks like a water worn topaz, which from MG could have a bluish color.

[Eduardoo]

Here is a somewhat better photo.

The SG is 3.51, so probably it is indeed a Topaz.

Thx for your comments.

Eduardo

[Antonio Alcaide]

It looks like my own water worn topaz from An Phu, Luc Yen, Yenbai Province, Vietnam.

The size, the colorless crystal, the termination, all of them fit. But of course it is only an educated guess. I have not an available pic now. I will try to post it tomorrow.

Regards.

[Peter Megaw]

There are another couple of easy and fun tests to try...that come from the gemstone world where they routinely have to discriminate irradiated blue topaz from true aquamarine beryl. Fastest and easiest is that topaz feels cool against your cheek or in your palm (this is true even for a small cut stone!) and beryl feels distinctly warmer. Also since topaz has a relatively high refractive index it will still have strong outlines if you look at it immersed in water, whereas beryl is enough lower that its outlines tend to disappear. Gemstone buyers use refractive index discriminating filters...either mounted in eyeglasses frames or hand held that do the same thing only better since the RI of the filter is higher than water. Cheapo versions look like a 35mm slide and sell for a few bucks. Of course the irradiators tend to make most of this unnecessary by zapping the topaz to blues that are inconceivable in natural material. If you happen to have a nucear reactor handy you could impart some color to this too!

[Matt_Zukowski]

Not that I doubt you Peter but how could one mineral "feel cool" and another "feel hot" if they have both been sitting around in the same environment? If both have temperatures equilibrated with the surroundings, wouldn't they both have the same temperature, and thus feel the same?

[crocoite]

That's how I was taught to determine topaz from quartz in rounded waterworn colourless stones. You hold the stone against the side of your face. They both start out cold, but the quartz warms up quickly. Try it. It works.

Regards
Steve

[GneissWare]

It due to differences in thermal conductivity.

This is a property that is used to differentiate real from faux pearls, and topaz and quartz, for example.

From the Quartz Page at:
http://www.quartzpage.de/gen_phys.html (link normalized by FMF)
"The thermal conductivity of quartz is higher than that of glass, so overall it can take a bit more, but it's still better to handle quartz crystals with care. In general, quartz feels colder than glass. The thermal conductivity is anisotropic and higher parallel to the c-axis."

On a gemstone forum, the question was asked about differentiating topaz from aquamarine, with the response:

"It's going to be impossible to differentiate between an aqua and a topaz just by looking at it. But here's a tip on I.D.-ing your stone. If you can get your hands on a diamond tester (thermal conductivity), test your stone. If it registers, it's a topaz. If it doesn't, it's an aqua. Beryl (aqua, morganite, emerald, etc) doesn't conduct, topaz does."

So this is a well known principle, and for some substances, a human is able to discern the difference as a sense of coolness and warmth. its quite evident in the case of pearls (real and faux) -- give it a try.

[Pete Modreski]

And this is the reason (or, one of the reasons) that diamonds are referred to as "ice". Diamond has, in fact, the highest thermal conductivity of any known material--even higher than that of the precious metals. As long as the stone is at a room temperature that is cooler than one's skin temperature, it should feel cool to the touch--quartz and topaz somewhat, and diamond much more so.

The apparent squarish/rectangular cross section of your crystal certainly suggests topaz; it does not look hexagonal as beryl would be.

Topaz should also feel noticably heavy compared to beryl and most minerals; specific gravities are, topaz about 3.5, and beryl about 2.7 which is very close to that of quartz, 2.65.

If one had access to a graduated cylinder into which the crystal could fit, you could do a crude (but useful) calculation of the stone's specific gravity by (carefully!) dropping it into water in the cylinder to measure its volume from the rise in water level, then weigh the crystal on a balance. The weight in grams divided by the volume in cc (1 ml = 1 cc) gives the specific gravity, and the difference between topaz and beryl should be large enough to distinguish easily. With experience, one should also be able to feel the greater density of a topaz crystal just by its "heft", if it's reasonably large.

[Matt_Zukowski]

oh - thermal conductivity. Interesting. thanks.

[John S. White]

Apparently Pete missed the comment that Eduardoo already determined that the S.G. is 3.51, which pretty much resolves the issue.