Finding these kinds of stones and cutting them in half reveals some amazing crystalline structures. This used to be part of our “family outing” going through gravel pits looking for agates, and other stones. We would use the “sunlight for discovery” to capture them.

Refractive indexes?

I’ll have to show picture on my blog of the collection in the future, as well as other crystals that I had acquired.

This does provide a further thoughts on Physical geodesy?

JAI, spyder:- Wow! You’re right! They’ve found Superman’s Fortress of Solitude! (Why did I not see that?) It’s in Mexico and not at the North Pole.. of course. Or maybe it is Bizarro’s.

Carl…. thanks for the tips! Everyone: – get to work!

Cheers,

-cvj

The planes of a crystal are defined by the directions that the crystal grows the most slowly. Directions in which a crystal grows quickly are eliminated in favor of other directions.

So if one grinds a crystal down to a spherical shape, and then uses that as a seed to grow larger, the sphere will change from a very complicated collection of faces to a more simple set of faces as the faster growing face directions grab material from solution. A sphere is a collection of an infinite number of faces, more or less.

The mineralogists classify crystals into 230 space groups. If you ignore the details of finite translations (which are impossible for the eye to detect but are easily detected in x-ray crystallography), these 230 fall into 32 point groups.

If there are enough faces, and there is not a serious degeneracy (for example a pseudocubic crystal), then one can determine the crystal point group by carefully examining a sufficiently complex crystal. Consequently, one should be able to determine the point group of a crystal by cutting out a sphere and letting it only partially regrow to its usual form.

A good example is salt or halite. Salt normally grows in cubes which could be any symmetry in the cubic system. But if a sphere were cut from salt and allowed to regrow a little, it should exhibit the hexoctahedral class, which means that one should be able to find crystal faces that exhibit a symmetry group of 48 faces. These are the hexoctahedral faces.

There are very few crystals that exhibit hexoctahedral faces. I’ll put some photos of one up on the web of fluorite growing in this fom if there is any interest.

All the various crystal growing materials should work with this method. With salt, I would try grinding out an approximate sphere, then washing it in water to dissolve microcrystals on the surface, then putting it in a slightly dissolving medium, and then slowly reducing the temperature so that the medium begins coming out of solution.

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