Directional Properties
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Author:  natascha [ Sun Mar 30, 2008 12:15 pm ]
Post subject:  Directional Properties

Directional properties.
I am supposed to answer a question in my gemology study to mention and briefly describe 6 directional properties possessed by gem materials.
I have written 5 but doubting the last;
Hardness, cleavage, DR, Pleochroism,surface markings.
Could I write isotropic or uniaxial is that a directional property?
Is the fact belonging to a crystal system not a directional property in itself?
Isn´t it so that all crystalline materials have directional properties?
As you can see I am doubting about "the limits" in the definition.
Looking forward to reading your answer.
Best Regards Ida

Author:  Doos [ Sun Mar 30, 2008 12:54 pm ]
Post subject: 


I would choose parting as the 6th. Anisotropy itself is not a directional property, more the cause of some of them.

Hope that helped.

Author:  JB [ Sun Mar 30, 2008 3:18 pm ]
Post subject: 

Growth zoning, twinning.

Author:  Taynara [ Sun Nov 02, 2008 4:08 pm ]
Post subject:  let see

1 Directional cleavage
3Directional Inclusions
4Directional hardness
5Double Refraction
6Crystal Habit

don't think that surface markings is a good answer for the fact that it would be inside directional inclusions.

Author:  agfa [ Tue Nov 04, 2008 7:29 pm ]
Post subject: 

hi ida

what a tricky question.

1- i think surface features would be an ok answer - trigons on diamonds are very definitely directional ; aquamarine crystals can have rectangular etch pits on the prism surfaces that are parallel to the c- axis. ruby crystals can show triangular growth hillocks on the basal pinacoids etc

2- what about RI - the RI of the extraordinary ray in a uniaxial crystal varies with direction.

3 - spectrum - DR gems often exhibit different spectrums in different directions - this is related to the pleochoism you have already mentioned and the cause of both is directional differential absorption of light.

i don't know if this helps but i hope it does.

best of luck with your studies

Author:  Taynara [ Wed Nov 05, 2008 3:36 am ]
Post subject:  hi there

well since the whole foundation questions are changing we'll have no problem in posting the ansewrs here. i got the question right but there is more that can be added.

1. Oriented cleavage.
Cleavage is the splitting of a gemstone along the direction of its crystal faces where atoms have a weaker bonding. This can occur only in crystalline material when a precise blow is given in a particular direction. The result of cleavage is a more or less flat plane with often a silky lustre. Cleavage is a reproducible property of a gemstone and can be done at any point of the cleavage direction. Cleavage can be prismatic, basal, pinacoidal, octahedral, rhombohedral and its quality can be described as perfect, good, fair, poor, none.

2. Double refraction:
Some crystal systems (anisotropic) have atomic structures that cause an incident ray of light to be divided into two rays travelling at different velocities in the same direction. This difference related to velocities is named ‘birefringence’. When the two rays change their directions of travel (are refracted) and move in different directions it is called ‘double refraction’.

3. Pleochroism:
Pleochroism is the change of colour in coloured anisotropic gemstones when viewed from different directions. When light enters the stone it will be split into two polarized rays. Each ray will be absorbed inside the gemstone in different amounts and will be differently coloured resulting in a ‘differential selective absorption’. Pleochroism can be described as strong, distinct, moderate, weak and none, and can be observed with a naked eye and with the aid of a dichroscope.

4. Oriented Inclusions:
Inclusions are foreign bodies, liquid, gaseous, solids, alone or in combination enclosed in the mass of a mineral. They include cleavage cracks, fracture, colour zoning, growth lines and included crystals of the same composition. Almost all gem materials whether naturally occurring or man-made, will contain inclusions.

5. Differential hardness:
Hardness is the ability to withstand abrasion by other materials. Due to various reasons some minerals are harder in one direction than in other direction. Kyanite is 5 in the Moh’s scale in the basal plane direction, while it is 7 in the prismatic direction. This phenomenon of varying hardness in different directions is named ‘directional hardness’.
Moh’s scale:
6. Crystal habit:
The habit of the mineral is defined by the shape in which it will usually occur in nature. Contrary to form, these are not always ideal shapes, although some might be. When we name habits, we change the suffix of the forms that make up the habit. For example, when the mineral has the form of an octahedron (as many diamonds have), we name the habit "octahedral". When a mineral is made up of different forms, we name the habit accordingly. Habits can be described as acicular, bipyramidal, botroyidal, columnar, dendritic, dodecahedral, euhedral, mamillary, mmassive, octahedral, prismatic, etc.

Author:  Barbara O. Ellis [ Wed Nov 05, 2008 10:47 am ]
Post subject: 

Wow! That's a lot to have to remember! Is there a Cliff's Notes available? :P

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