How To Orient Sapphires With Silk Inclusions


Silk inclusions in sapphires are microscopic rutile crystals that developed inside the gemstones. While the appearance of “silk” can lower the value of sapphires, the rutile crystals can also create the asterism or “star sapphire” effect. “Rutile in sapphire, Unknown Field of view=2.1mm • Depth of field=1.175mm” by Danny J. Sanchez.
Silk inclusions in sapphires are microscopic rutile crystals that developed inside the gemstones. While the appearance of “silk” can lower the value of sapphires, the rutile crystals can also create the “star stone” or asterism effect.  Star sapphires are highly prized. “Rutile in sapphire, Unknown Field of view = 2.1mm • Depth of field = 1.175mm” by Danny J. Sanchez.

Question

I’m about to cut a sapphire but I have a question about something I haven’t seen before. I think they could be silk inclusions but I’m not sure. This 11-carat sapphire is from the Umba River valley of Tanzania. The c-axis is baby blue, the other is soft lavender. I acquired this gemstone many years ago. Now I hope to get a good return once I facet it.

I polished a window into the stone and saw an unusual inclusion pattern. It was like facets running in a criss-cross crystalline pattern. You can’t really “see” them, but they reflect/distort the light pattern slightly. They’re not tiny flecks. They run the whole length and width. The inclusions look best along the baby-blue c-axis with a 10X loupe. You can just make them out with the naked eye. Along the lavender axis, you can’t really make them out unaided. With a loupe you just see tiny lines.

I thought this gem was perfect for a Barion emerald cut with the baby-blue axis. Now I’m not so sure. Would it be better to go with the lavender axis, where the inclusions are not as noticeable?

I always thought silk inclusions would be more obvious and make the stone opaque. Or is this what is referred to as the velvet-like texture of Kashmir sapphires?

Theo

Answer

You just discovered silk inclusions. This is why most corundum gems are routinely heat treated. Silk inclusions are often hard to see before polishing and can take the stone down several grades. However, I wouldn’t recommend having it treated. Most treaters only do batches, and each gemstone is different. It takes a fair amount of experimentation to find the right combination of stones for a batch. To have an individual piece treated is very chancy.

The Umba River valley in Tanzania is a source of fine sapphires in a wide variety of colors. “Umba Sapphires” by Rasa Rocks is licensed under CC By 2.0
The Umba River valley in Tanzania is a source of fine sapphires in a wide variety of colors. “Umba Sapphires” by Rasa Rocks is licensed under CC By 2.0

As far as which axis to go with, do you have a polished window on the baby-blue axis? If so, put a tiny drop of oil on it. Then, hold it under a downward facing lamp. Look carefully to see if there is a star on the oil. (It might be very hard to find). If you find a star, your best bet would be to cut a star gemstone. A transparent, blue star sapphire is a prize! If you don’t see a star, I would go with the direction of best transparency. The silk inclusions will give a muddy color to the stone and it can look a lot worse after faceting.

Silk inclusions can be microscopic to opaque. The real trick is judging if a sapphire has enough silk to star or not. That’s something that only experience can teach.

What you’re describing doesn’t sound like velvet. Kashmir sapphires are full of invisible inclusions that leave them a bit less than fully transparent.

Donald Clark, CSM IMG

A Follow-Up Question

Thanks for the information. I have some more questions. I tried the oil trick but I’m not sure if it has a star. It may have an eye. The obvious reflecting silk inclusions are on a 45º angle with the 8mm axis. There are 19 lines that I can count with a 10X loupe. Maybe that’s not enough to make a good star?

Theo

Answer

OK, if you’re looking down the c-axis, the silk inclusions will form a hexagon. If you’re only looking at part of it, you will see it meeting at 60º angles. If you orient the top of the stone to the c-axis, you will get a star. If you orient it to one of the long axes, it will cat’s eye and not be worth nearly as much.

A stone can star with only microscopic silk inclusions, not just what you can see. That is why there are transparent star stones. Finding a star with the oil trick is very hard, but always try it first. The next step is to gently smooth and round out all the sides of the stone. Try not to remove anything that wouldn’t be cut away anyhow. Give it at least a 1200 finish, then coat it with oil. Now take the stone out into the sun (preferably) or under a bright light. Look at it from several directions. If the stone will star, you will see a partial star on one of the rounded surfaces. Orient the stone in that direction. What you see will also give you an idea of the intensity of the star. If it’s weak, the gem probably isn’t worth cabbing.

Donald Clark, CSM IMG

“Star-Saphire.” Public Domain
“Star-Sapphire.” Public Domain