Before your face contorts with trying to rationalize that, let me remind you that gemstone nomenclature is about fashion, not minerology, and you can read my other posts on this subject to learn more. What a gemstone is known as in the fashion jewellery industry can mean many different things, and sometimes, well, let's face it, sometimes it's fake.
Black onyx doesn't exist in nature. There are very dark charcoal grey onyxes and very dark charcoal grey banded agates (I'll come back to that later) but not solid black. Not really black. It just doesn't come out of the ground like that. If you want a "natural" solid black gemstone, you should choose instead obsidian or jet. But not all obsidian is black either, and even when it is, if you hold it up to the light you will see it's really a very dark brown. And jet is coal, actually. So it really is black, but it's not a gemstone. And, before we muddy the waters even further, French Jet is...glass. But then, so is obsidian...it's volcanic glass. Have I lost you yet? Oh dear! You begin to see why this needs a whole blog.
If you want the look of solid black, choose any one. But understand their limitations.
Most black onyx on the market has been dyed to make it blacker. This is what the customer wants, solid black, no white or grey bands. I buy my black onyx undyed, and then I select from it the pieces with minimal banding and "off" colour. The blackest ones I sell as black onyx, the banded ones I sell as striped or banded black onyx. Sometimes the blackest ones have a very faint banding, and sometimes it only shows up in really bright light. Inevitably, some people object to this. Personally I think it's lovely, and the majority of my customers do too, they are delighted with this subtle "imperfection" and probably surprised too.
The question then, is how you, who are not a gemstone expert, can tell the difference.
First, I must ask you why you need to. If you like what you see in front of you, presumably you bought it because of its appearance. These are not valuable gemstones, and not rare, there is no investment motive to buying them. If they look good, and serve their purpose, does it really matter if you got black onyx instead of obsidian, or vice versa? I can understand you being upset if it turns out to be glass, but that's much easier to determine, and you know, even then, black glass is a modern miracle, and it can cost as much if not more to produce than regular grade onyx or obsidian. It's only made because of its regularity, i.e. you can guarantee the end result.
The exception would be those who follow crystal healing practices, however I would suggest that if they can't tell that obsidian isn't crystalline, they're probably charletans anyway.
Glass or jet is easily recognized by a simple scratch test, and this will permanently mark your stone, so either do it on the back, or be ready to accept the consequences. Scratch tests, you may remember from school, are where a harder mineral will scratch a softer one, with the harder one remaining unmarked. To tell between glass and jet, float it. Glass will sink, jet (which is related to amber) is light, and will float.
Onyx and obsidian also sink in water, but will not scratch easily, and that's despite obsidian being a type of glass. It is much harder than, for example, bottle or window glass, and will not be scratched by these.
When it comes right down to it what we are really asking here is how to tell between onyx and obsidian.
#1 Obsidian is never banded. If you see any hint of a stripe when it is held up to a strong light, it's not obsidian. It may however have tiny "bubbles" if you have a jeweller's loupe or other strong magnification available.
#2 If you actually have a powerful microscope handy, you can see an obvious crystal structure in onyx. Obsidian, being a type of glass, has no crystal structure whatsoever.
#3 If you have both, comparison can be done. Obsidian is naturally brighter, again, because it is glass.
#4 If the two pieces are the same size (for example, two 8mm beads), the onyx will be heavier.
#5 If you only have one piece, nothing to compare it to, and see no bands, it is not easy to tell, without experience. It is probably safe to say that if you hold it up to the sun (carefully, blocking the light fully) and it still looks opaque black, you do not have obsidian. What you have is also probably dyed.
However it may not be black onyx. It may be black agate.
What's the difference? Well......Onyx is a type of agate. It is therefore always correct to call onyx agate, but not necessarily correct to call agate onyx. It's a subset, not a synonym.
Onyx has finer crystals than agate, they are called microcrystalline. This results in the familiar bands being very regular. If dye has been used, and bands are hard to see (to the naked eye they may not be seen at all) then telling onyx from "just agate" is virtually impossible without a microscope. If bands are present, you can use those as a guide. Banded agate is still lovely, it's just not quite as "fine" as onyx.
I'm sure you'll be browsing Google images now, so I'll leave you to it :)
When it comes right down to it what we are really asking here is how to tell between onyx and obsidian.
#1 Obsidian is never banded. If you see any hint of a stripe when it is held up to a strong light, it's not obsidian. It may however have tiny "bubbles" if you have a jeweller's loupe or other strong magnification available.
#2 If you actually have a powerful microscope handy, you can see an obvious crystal structure in onyx. Obsidian, being a type of glass, has no crystal structure whatsoever.
#3 If you have both, comparison can be done. Obsidian is naturally brighter, again, because it is glass.
#4 If the two pieces are the same size (for example, two 8mm beads), the onyx will be heavier.
#5 If you only have one piece, nothing to compare it to, and see no bands, it is not easy to tell, without experience. It is probably safe to say that if you hold it up to the sun (carefully, blocking the light fully) and it still looks opaque black, you do not have obsidian. What you have is also probably dyed.
However it may not be black onyx. It may be black agate.
What's the difference? Well......Onyx is a type of agate. It is therefore always correct to call onyx agate, but not necessarily correct to call agate onyx. It's a subset, not a synonym.
Onyx has finer crystals than agate, they are called microcrystalline. This results in the familiar bands being very regular. If dye has been used, and bands are hard to see (to the naked eye they may not be seen at all) then telling onyx from "just agate" is virtually impossible without a microscope. If bands are present, you can use those as a guide. Banded agate is still lovely, it's just not quite as "fine" as onyx.
I'm sure you'll be browsing Google images now, so I'll leave you to it :)
