What does the term "open" stone mean?

[Gempassion]

Hi,

I have come across the term "open" stone or "open" color... Does it mean vivid or bright?

 

[chrono]

An easy to understand example of open colour is to look at most blue and green tourmalines. Many have a closed C axis (which show darker colour on both ends after being cut, and when viewed through the C axis) with the prized colour only in the middle of the stone. Therefore, if a stone has open colour, it means that it doesn’t have this dark ends like a closed C axis stone which results in an even colour throughout the stone.

 

[Arkteia]

Once I bought a closed C-axis tourmaline on the ebay. The vendor warned that the photo was taken under UV light. And it was not a stone, just a chrystal; I bought it for my own education. All I can say - do not buy a closed C-axis stone if you plan to set it in a piece of jewelry. Mine looks like a burnt match with dark-greenish tinge. I would have posted it here but it doesn't even photograph like a stone.

 

[PrecisionGem]

"open color" is often a term used to describe a brighter stone. When buying rough, I'll see a parcel of rhodolite garnet that's a bit dark, and then the dealer will pull out some Umbalite, that is much lighter, and say; "look at the open color on these!" I think at least in terms of rough, open color means a brighter stone, that should cut with out much extinction.

 

[Michael_E]

All I can say - do not buy a closed C-axis stone if you plan to set it in a piece of jewelry.

This is not always true, usually, but not always. Blue and green tourmalines often have closed "C" axes, but can be very attractive if they are long enough and cut so that light doesn't cross the dark axes very often. With the appropriate crossed bar or checkerboard crowns and very steep pavilion ends these stones can be spectacular. The attached image is of a tourmaline which had a very dark "C" axis, but a fine, bright green color on the A-B axes. I cut the crown very short, (so short that I used a checkerboard rather than a crossed bar). The stone was set into a pendant for my step mothers birthday and she seemed to like it. Very little darkening on this stone in medium to bright diffuse lighting. With closed "C" stones cutting is more critical than with "better" roughs, but if it's done right the end result can be a very nice stone at a very moderate price.

 

[Sagebrush]

I've heard the term "open color" used mostly to describe a sapphire with good crystal. Usually it refers to a stone that lacks the heart of darkness you sometimes see below the table. A lot of cutters in Asia will cut a window to negate the effect. Its one of those terms that lack a really precise meaning.

 

[Gempassion]

Thank you Chrono, Crasru, Gene, Michael, and Richard,

Your divers explanations have clarified the term (but, I supposed Richard is correct in saying that it's one of those nondescript terms).

Gempassion

 

[Arkteia]

All I can say - do not buy a closed C-axis stone if you plan to set it in a piece of jewelry.

This is not always true, usually, but not always. Blue and green tourmalines often have closed "C" axes, but can be very attractive if they are long enough and cut so that light doesn't cross the dark axes very often. With the appropriate crossed bar or checkerboard crowns and very steep pavilion ends these stones can be spectacular. The attached image is of a tourmaline which had a very dark "C" axis, but a fine, bright green color on the A-B axes. I cut the crown very short, (so short that I used a checkerboard rather than a crossed bar). The stone was set into a pendant for my step mothers birthday and she seemed to like it. Very little darkening on this stone in medium to bright diffuse lighting. With closed "C" stones cutting is more critical than with "better" roughs, but if it's done right the end result can be a very nice stone at a very moderate price.

Would it be possible to see a photograph of the pendant? I am just thrilled, the stone looks unusual.

 

[Michael_E]

Unfortunately not Crasru. The recipient of that pendant has a birthday which is very close to Christmas and I just ran out of time in getting it to her, so had to forgo the pictures. That coupled with the fact that she might have some trouble getting a good picture makes seeing that a very remote possibility, sorry! I'm a little surprised that more people haven't posted images of closed axis stones, some that I've seen are very nice.

 

[Swala Gem Traders]

I thought I may add a word as the term 'open color' is often used in "our" part of the world. It's a term which my brother Mark uses a lot as a matter of fact. I'll confirm what Gene said: the term is used for gems which are often dark. For instance, rough rhodolite is often dark. If I can use an image which is technically incorrect, such gems usually 'keep the color inside'. Therefore when a rhodolite is light (which is not that often with bigger pieces), the term 'open color' is applied.

It hope that helps.

 

[chrono]

Michael,
I’ve read and also seen for myself that a checkerboard cut seems to help a closed C axis tourmaline appear much brighter and attractive to the eye. My guess for not seeing more great examples of well cut closed C axis tourmalines on PS is that most are not fans of checkerboard cuts for one reason or another, myself included.

 

[PrecisionGem]

It's not the checker board that helps with a closed axis tourmaline, but that the closed ends are cut at very steep angles, and the the facets on the open ends run parallel or almost parallel to the stones open axis. The opposed bar cut that M.E. showed is a perfect example of such a cut. I find this cut works better than an open axis stone. It adds more contrast to the facets as the reflect light. These cuts look like LED lights lighting up and down the length of the stone.

 

[Gempassion]

Gene: the LED effect sounds very intriguing.

Thank you for the explanation Swala (I'm happy to know that the term applies to gems that are normally dark).