bsky4463
fully equipped rock polisher
Member since September 2013
Posts: 1,696
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Post by bsky4463 on Mar 8, 2014 22:53:15 GMT -5
Starguy posts of Idaho garnets has me fired up so go back to the USFS site this summer. Saw this blurb in a G&M club newsletter and caught my attention - learn something every day.... "Aasterism is an optical phenomenon displayed by some rubies, sapphires, and other gems. A reflective area in the shape of a "star" appears on the surface of a cabochon cut from the stone. The Star-effect or "asterism" is caused by the dense inclusions of tiny fibers of rutile. The stars are caused by the light reflecting from needle-like inclusions of rutile aligned perpendicular to the rays of the star. Since impurity is always what causes creates the star gemstones, they are almost never completely transparent."
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39don
starting to spend too much on rocks
https://www.etsy.com/shop/DonsLapidaryArts
Member since February 2012
Posts: 225 
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Post by 39don on Mar 8, 2014 23:31:15 GMT -5
A friend of mine from Kings Mountain, NC has a bunch/lbs of garnets he collected some years ago. They are real gemmy looking except for the fractures. They will have a star but mostly all are 4 rays. He has one star garnet that is 1 1/2" to 2" long X 1" to 1 1/4" wide X 1/2" to 3/4" thick. It looks like a 2 sided oval cab. When he holds it in the sunshine one of the rays, side to side, appears to rise up above the stone's surface. It's high enough it looks like I could stick my finger under it. WEIRD His comment about the fractures "they give the garnets character" and believe me that large oval has lots of character. A four ray star cab I cut last year......  39don |
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bsky4463
fully equipped rock polisher
Member since September 2013
Posts: 1,696
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Post by bsky4463 on Mar 9, 2014 10:50:06 GMT -5
Rutile (ˈro͞oˌtēl)
-the most common natural form of titanium dioxide - a black or reddish-brown mineral typically occurring as needlelike crystals.
-has among the highest refractive indices at visible wavelengths of any known crystal and may contain up to 10% iron and significant amounts of niobium and tantalum.
-derives its name from the Latin rutilus, red, in reference to the deep red color observed in some specimens when viewed by transmitted light.
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Post by Starguy on Mar 9, 2014 12:48:47 GMT -5
He has one star garnet that is 1 1/2" to 2" long X 1" to 1 1/4" wide X 1/2" to 3/4" thick. It looks like a 2 sided oval cab. When he holds it in the sunshine one of the rays, side to side, appears to rise up above the stone's surface. It's high enough it looks like I could stick my finger under it. WEIRD and 39don; Asterism is a strange phenomenon. My experience is that if you cut high domed cabs, the star appears to sink below the surface of the stone. Flat dome cabs appear to have the star floating above the surface of the stone. This appears to be the case with garnet, corundum or Linde material. I haven't cut any star rose quartz although I have some nice gemmy rough squirreled away somewhere. Most of the rose quartz I have seen were either spheres or tumbled with pretty high domes. My theory about the sunken versus floating stars is similar to looking at stereo photographs or 3D movies. Because the stars move around based on the point of view or the angle of the light source, each eye sees something slightly different. This tricks yor brain into seeing a floating or sunken star. I have cut some cabs with two domes on them, with a valley between the domes. These cabs showed two distinct stars on the top of the cab. With garnet, there is no C-axis like there is on corundum or rose quartz. If you polish a bead or sphere of corundum or rose quartz, it will show two stars. Garnet sphere/beads will show multiple interlinked stars around the entire stone. later Brent |
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Post by gingerkid on Mar 9, 2014 12:50:29 GMT -5
Thanks for sharing this info with us, bbsky463. I enjoyed reading that starguy's star garnets got you fired up.
Very pretty star garnet cab, Don! I didn't know that the NC almandine had asterism-pretty cool-and thanks for posting! That's weird that one of the legs appears to be reaching out from the cab's surface when you look at in the sunlight. optical illusion?
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39don
starting to spend too much on rocks
https://www.etsy.com/shop/DonsLapidaryArts
Member since February 2012
Posts: 225
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Post by 39don on Mar 9, 2014 15:17:11 GMT -5
Very pretty star garnet cab, Don! I didn't know that the NC almandine had asterism-pretty cool-and thanks for posting! That's weird that one of the legs appears to be reaching out from the cab's surface when you look at in the sunlight. optical illusion? Come to think of it by what srarguy said about the flatter the crown the more projected the star may look. The large oval my friend has has a flatter crown than normal due to the largeness of the garnet. I have a few more of his garnets that have proven stars after tumbling. I will cab one with a flatter surface and see what happens. Thanks starguy, 39don PS. I have found these NC star garnets have a star radiating at each corner of the crystal. Some stars are more defined than others. This friend said he has located an outcropping of garnets that have a 6 ray star but it's on private land. He's trying to get the owner's permission to dig, no success yet. DD |
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