thanks for all the info ...can u help me with gems that have alexandrite effect.
Colour change stones? There's a *lot* of information here. Search the GO archive by enterring colour chanage into the on line search box in the left hand panel of the page you are looking at. Also, check out the Gemology Project (l/h panel again).
You can always come back for anwers to questions that you can't find in those places
Joined: Wed Aug 24, 2011 11:48 am Posts: 584 Location: Odenton, Maryland
Exactly. Impurities as chromophores don't affect hardness, nor do inclusions really, as long as they're small enough. Now, a band of galena in your agate cab will affect hardness, because you have a big chunk of lead in your quartz that will undercut because it's so soft. But in gemstones, you're dealing with a single mineral, and any sizable impurities (like matrix) are cut out.
Joined: Wed Aug 24, 2011 11:48 am Posts: 584 Location: Odenton, Maryland
ttnt779 wrote:
i'm wondering if the hardness of gemstone is decided by the main element and impurity of the gemstone
It's more about the crystalline structure than the component elements. For example, garnets generally have the same formula (X3Y2(SiO4)3). Almandine (Fe2+3Al2(SiO4)3) has about the same hardness as Pyrope (Mg3Al2(SiO4)3).
Even if you consider silicon (Si), or even Si0x, to be the main element, quartzes (SiO2) are a bit softer than most garnets.
It's probably fair to say that structure is more important in a general sense, though. The most extreme change from a structural difference I can think of is diamond vs graphite, while the biggest difference within a structural group by composition is andradite (6.5ish) vs. YAG (8.5ish). That's still orders of magnitude on most other scales but still. Can anyone else think of a more extreme example? There's certainly a nice bit of variation in the clinopyroxenes (~5.5-7), and no doubt the amphiboles do weird things as per usual.
sapphires, from my area, can have dramatically different hardness within one piece of rough. No visible reason, but it can take minutes to cut a facet, using 1500 lap. The next one will overcut in a split second. Yellows are often vexatious like this! Barry.
Barbra, it is well known amongst gemcutters on the Qld fields, that our sapphires have dramatic hardness changes, within some stones. There is absolutely no way to tell where they are, until you start faceting. I have tried to find a reason, by carefully loupeing the gem, when I hit one of these areas. The only thing I've found, so far, is there is often a colour zoning associated with the extra hardness. This can be very, very subtle. Another time I find this, is when cutting synthetic material, and using the "rind" of the boule, in the design. This afternoon, cutting some vanadium dopped, colour change corundum, I placed the "rind",(deep purple), under the pavillion end facets, on an oval. The facets took about 20 times longer to set, than any of the others, using the same 1500 lap. Earthling will have more to contribute on this subject, I'm sure! Barry.
thanx and how accurate is a presidium gem tester..i have recently purchased one...can i depend on its results...i am a collector...is there any other instrument which will help me recognise gemstone rough....
I have basically been using [url]Gemlogis testers[/url] and they are really accurate. I would recommend yoiu to try one of those.
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