I am new in the gems world and i have a doubt about the lessons i ve been reading. One lesson says that a star in a stone is because when the cavities of crust go on and of some different mineral might form and a star happens when the new layer doesnt bond well with the old one and other lesson a star and cat eyes is a special features of stone that is an impurity of rutile inclusion present on them could someone help me?
Hello! First of all, sorry for my poor english.
The most common is that diffraction of light in alternating mineral layers (impurities?) of a cristal generates a phenomenon called Adularescence. Asterism (chatoyancy) is usually caused by rutile inclusions.
(From some IGS texts:
"Asterism refers to a star effect. The most common star stones are rubies, sapphires, and diopside, the “black star of India.” Somewhat rare are star garnets and quartz stones. Very rarely, you will find another mineral that will star.
The star is similar to a cat’s eye. The effect is caused by light reflecting off internal, silk-like inclusions (usually rutile). However, in a star stone, the inclusions are crossed rather than straight. Corundum and quartz crystalize in the hexagonal system and develop six-rayed stars. Garnet and diopside have four-rayed stars. You’ll rarely find star quartz in jewelry. The star is faint and most often seen in spheres. Even then, good lighting is required to see the star clearly.
Chatoyancy is a form of sheen. Sometimes, it’s just a silky glow as in some tiger’s eye and charoite. When it’s highly organized, you get a cat’s eye. Cat’s eyes are the result of light reflecting off silk-like inclusions. These are usually minerals, but internal fractures and veils will occasionally produce the same effect.
Labradorescence is a flash of color caused by the interference of light due to repeated twinning. Where adularescence comes from inside the stone, labradorescence is primarily on the surface. This is effect is often wrongly called play of color. That term belongs exclusively to opal.
“Schiller” is a form of aventurescence. It’s a glow caused by light reflecting off inclusions. However, schiller is usually much more striking than other forms of aventurescence. It’s most commonly found in sunstone, where the inclusions are hematite and, in Oregon sunstone, copper.
Adularescence is due to diffraction of light as it hits thin, alternating layers of orthoclase and albite within the gem. "
(Search for: “phenomena in gemstones”)
Hello Fufumahfouz,
Can you remember which lesson states that stars happen when the new layer doesn’t bond well with an old layer?
Hello Fufumahfouz,
I think I may have found the source of confusion.
Star stones are caused by rutile inclusions, this is discussed in two syllabus articles, “Gem Formation” and the “Asterism” section of the “Phenomenal Gems” article. Both articles also discuss twinning or parting in star sones. This may cause weakness in the stones when layers of the crystal do not bond well.
What causes twinning is that crystals may not grow all in one continuous process. Their growth can start and stop, as the space in which it’s forming underground is opened and closed and the elements for its growth may no longer be present, or new elements may enter the space. Also, conditions like temperature and pressure may change when the space is closed and growth resumes.
In the Gem Formation article, the text explains that this stop/start growth process is also one of the ways inclusions are formed in gems. This may explain why rutile crystals, a distinct gem species, can grow inside sapphires. So, this interrupted growth process may cause either inclusion formation or twinning, and in some cases, both.
Inclusions and twinning are two distinct things that may have the same cause.
Unfortunately, there is a caption for a star stone pic in the Gem Formation article that mixes this up. It reads: “The star shape in this ruby is a result of new layers not bonding with old layers within the gem.” This is not correct. Rutile inclusions cause the star stone effect, not twinning. I’ve removed that caption.
I hope this clarifies the issue. If you were referring to other articles, please let me know.
Like anything with gems, there are a number of reasons they may have a star appearance. The lessons you are reading are correct and while they seem to contradict in the thinking of “it only happens one way”, it could become confusing. Keep your mind open to things happening in lots of ways. Color for example can be cause by Allochromatic, Idiochromatic, Color centers and a few more. And the same trace elements in different gems can cause different colors.
Although rutile inclusions are the most common way star stones are formed, there are indeed other causes. For example, titanium silk in hexagonal corundum can also cause asterism. As for parting, in yet another article, Donald Clark writes:
Well-cut star and cat’s eye gems are a delight to the eye and great fun to cut. These phenomenal stones display asterism and chatoyancy, respectively. Both effects are the result of light reflecting off inclusions within the gemstones. Although silk (fine included crystals) is the most common cause of these visual phenomena, any inclusions that reflect light in parallel lines can create these displays, including hollow tubes, feathers, and veils.
So, it seems there’s more than one way to skin a cat’s eye.
Does anyone have pics or information on any stones with asterism caused by parting?