214:) present in them. The star-effect or "asterism" is caused by the difference in refractive index between the host material and that of the dense inclusions of tiny fibers of rutile (also known as "silk"). Rutile causes the relative bright relief of a star in a host material such as corundum, which has a refractive index between 1.760 and 1.778, much lower than that of rutile. The stars are caused by the light reflecting from needle-like inclusions of rutile aligned perpendicularly to the rays of the star. The star-effect may be also caused by the inclusions of
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218:. In black star sapphire hematite needles formed parallel to the faces of the second order prism produce asterism. Some star sapphires from Thailand contain both hematite and rutile needles forming a 12-ray star.
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within which was the appearance of a star shining with the light of the moon. However, since rutile is present in most common star gemstones, these are almost never completely transparent.
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with near uniform impurities which is bluish-grey and milky or opalescent, which when lit has a star of six rays. In the red instance stellate reflection is rarer; the star-
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253:, is the result of light transmitted through the stone. In order to see this effect, the stone must be illuminated from behind. Rose quartz also exhibits epiasterism.
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D. S. Phillips, T. E. Mitchell and A. H. Heuer,"Precipitation in Star
Sapphire I: Identification of the Precipitates, Phil. Mag. A, 1980, v. 42, N0. 3, pp 385–404
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inclusions within the stone's crystal structure. A common cause is oriented sub-microscopic crystals of
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is among the most valued of "fancy stones". Other examples are star-
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16:"Star ruby" redirects here. For the grapefruit variety, see
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Star-stones were formerly regarded with much superstition.
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A distinction can be made between two types of asterism:
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Asterism is generated by reflections of light from twin-
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Nature's
Building Blocks: An A-Z Guide to the Elements
107:A gemstone that exhibits this effect is called a
27:Asterism on the surface of a blue star sapphire.
155:may display four-rayed or six-rayed asterisms.
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35:A purple-pink star sapphire displaying
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206:and rubies display the property from
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225:'s example is consistent with a
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100:(i.e. shaped and polished, not
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127:The archetypal asteria is the
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264:Isomorphism (crystallography)
18:Grapefruit § Star Ruby
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68: 'star' and
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175:within the gem mineral.
336:Oxford University Press
316:Encyclopædia Britannica
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330:Emsley, John (2001).
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115:. The best known is
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251:rose quartz
241:Epiasterism
159:Description
76:star-shaped
412:Categories
334:. Oxford:
275:References
269:Chatoyancy
247:Diasterism
109:star stone
47:(from
387:cite book
379:226280870
361:Gemmology
227:moonstone
184:sapphires
141:Sri Lanka
123:Archetype
87:refracted
83:reflected
418:Gemology
258:See also
216:hematite
200:cabochon
192:diopside
169:acicular
165:lamellae
149:diopside
133:corundum
97:cabochon
91:gemstone
45:asterism
37:asterism
311:Asteria
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198:when a
153:garnets
113:asteria
102:faceted
89:from a
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212:rutile
196:spinel
194:, and
188:garnet
180:rubies
173:rutile
231:India
145:topaz
80:light
63:astḗr
56:ἀστήρ
51:
393:link
375:OCLC
365:ISBN
340:ISBN
137:ruby
71:-ism
313:".
111:or
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