I believe that a main article in the IGS website on “polariscopes” has a incorrect definition and use of twinning. My understanding is that a “twin” is a new direction in the growth of a mineral at a specific time in its growth… having a completely unique Miller Index value. The main article currently listed seems to define a twin as a stoppage and then resumed growth of a crysal as to add a layered appearance to the gem. Please read the posted article and then let IGS or me, know what your view of “twinning” is… Thank you… Dion
Hi Dion,
That is an interesting question. And you are correct, the Miller and/or Weiss Indices will always have a unique value for a twinned crystal, no matter how the twinning occurs.
There are several articles regarding polariscopes here at IGS. The ones I have reviewed so far haven’t specifically addressed the question, so I am not sure I have read the right article. Can you point to the specific one, please?
Thanks!
-Troy
Hello Dion,
I think what Donald Clark intended in his discussion of polariscopes and gemstone twinning is that some new layers of crystal growth have a new orientation of growth. I think he presented that more clearly in his article on gem formation. Unfortunately, the passage in the Polariscope article incorrectly states: “Some crystals will continue to grow as a whole, without any division. Others will grow in layers, one on top of the other. This is known as twinning.” You are correct, the “layering” of new crystal growth is not what is known as twinning. The new crystals must grow in a different direction for this to qualify as twinning. I’ll correct that article. Thank you for pointing this out.
Pedro
IGS Admin