12 votes
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Why do crystals, like quartz and diamonds, form in different colors?

The reason minerals like quarts and diamonds vary in color is generally caused by the chemical elements involved while the crystal is being formed. Chemicals Different colors can be created by ...
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  • 2,802
12 votes

Is this ice cover real - and what circumstances are required to make it?

Yes, it is real. Whoever took the photo, congratulations on a very fine image. I have never seen this texture on such a scale, but something similar can be achieved in the laboratory by creating a ...
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12 votes
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Why there are no sharp corners in nature?

The premise of your question is entirely wrong. There are many examples of sharp corners in nature, the most obvious being well formed crystals. Pyrite is a classic example that was mentioned by @...
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  • 2,211
12 votes
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What determines a mineral's hardness?

The hardness of minerals is diagnostic because the hardness is determined by the strength of bonds and the structure of the mineral lattice. Hardness is basically the stress required to create and ...
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  • 3,780
10 votes
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How and where do double-terminated crystals form?

Double-terminated crystals can from by crystallizing from a melt. The crystallization nucleus has to float freely in the magma chamber. As long as no other crystals obstruct the growth the crystal ...
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  • 3,876
9 votes

Is this ice cover real - and what circumstances are required to make it?

This picture was taken in what seem to be a small pond, of very calm water. It seem that in ideal conditions, implying but not limited to : absence of winds clean water eg: no nuclei to provide an ...
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  • 2,623
9 votes

Why do crystals, like quartz and diamonds, form in different colors?

I'd like to elaborate of the Chemicals issue of Azzie Rogers' answer. You can divide the chemical coloring into three main parts (there may be more, but these are the important ones): Inclusions A ...
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8 votes
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What is this fossil? What is this crystal?

Yes, this is a rather nice find; an ammonite of the Jurassic in central Europe. Given the size of the specimen, and its appearance, this appears to be of the species Cardioceras, or Perisphinctes. ...
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7 votes
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Why are there no crystals with 5-fold symmetry?

All unit cells are parallel-sided hexahedra. These are six sided shapes with parallel opposite sides. Their three principle angles may or may not be 90 degrees. And the three side lengths may or may ...
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  • 3,823
6 votes

How can the 'crystal cleavage' of apatite have a four-digit number?

This is not a four- digit number but four separare numbers called Bravais-Miller indices. Bravais-Miller indices descrive the orientation of a crystal plane relative to the symmetry axes of a crystal,...
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  • 2,491
5 votes

Does Xenon really covalently bond to oxygen within quartz?

Not sure this is appropriate for Earth Science SE (Chemistry SE would be a better fit), but the answer is "maybe". quoting from the same Wikipedia article: Three oxides of xenon are known: ...
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5 votes

What kind of minerals have no cleavage plane?

I'm pretty sure a more rigorous answer deserves to come along, but I can give a simple overview of some of the important factors. Cleavage planes have to do with bond strength and bond geometry. If ...
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  • 541
5 votes

What determines a mineral's hardness?

There are some subtleties that I'd like to add, in addition to Mark's answer. When talking about the hardness of a mineral, the nature of the chemical bonds in the crystal structure (e.g. covalent vs ...
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  • 22.2k
4 votes

Why does the colour of amethyst fade if exposed to too much light?

The color of a mineral can be caused by a variety of mechanisms. This is also true of amethyst, which is a variety of quartz ($\ce{SiO2}$), and can be found in many colors. The major factors ...
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  • 3,780
4 votes

Why are there no crystals with 5-fold symmetry?

Your question is much more complicated from mathematical point of view than it seems to be. First, I'll start with a nice photo: (source: Wikipedia). What you see is really a photo and it is almost a ...
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4 votes
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What's the difference between cleavage and fracture?

They are not synonymous. Cleavage means breaking along planes defined by crystallographic directions. For example, cubic crystals like halite, NaCl, often cleave along directions that follow the ...
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  • 4,319
4 votes

When diamonds "migrate" from deep underground to the surface, do they maintain pressure inside when there is no more pressure outside? If so, how?

The question is in regard to pressure confining a rare, deep-mantle formed mineral visible within a diamond inclusion. The pressure on the inclusion within the diamond crystal is really the pressure ...
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3 votes
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What does the unit cell of petitjeanite look like?

Should one assume that the mineral petitjeanite and the chemical discussed in the recent Chemical & Engineering News article Photocatalyst shreds drinking water contaminant PFOA are probably the ...
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  • 146
3 votes

An oblique-square-prism crystal?

If "oblique square prism" is this, where the top and bottom planes are parallel, then yes, there are. Carbonates (members of the Calcite and Dolomite, but not Aragonite, groups) have perfect cleavages ...
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  • 348
3 votes

A few questions about glass

Q1: A volcano consists of many lava flows that occur over a long period of time. In general, only very few of these lava flows have the right composition and the right cooling history that are ...
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  • 161
2 votes

A few questions about glass

In particular, in season 7 of Game of Thrones, the island of Dragonstone is revealed to contain a veritable mountain of obsidian - is such a thing possible in the real world? I would have thought that,...
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2 votes

Is there any covalent crystals that can be easily synthesized?

Silicon carbide is covalent and can be synthesized at atmospheric pressure by the Acheson process. Temperatures may actually be somewhat higher than your range but are easily achieved by the process. ...
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  • 4,319
2 votes

Why does halite have perfect cleavage at the (110) plane?

I think the commenters have identified the issue. Halite does not have perfect cleavage along the {110} plane. As recorded in Mindat, Halite has perfect cleavage alone {100}, {010}, and {001}. This ...
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2 votes

How and where do double-terminated crystals form?

Double terminated crystals normally form in free floating pockets of liquid that slowly evaporated, leaving perfectly formed crystals with terminations on both sides. Unlike usual quartz formations ...
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  • 1,622
2 votes

Can the formation of gypsum evaporites (sand roses) be simulated in the lab?

Yes, it has. Cody & Cody (1998), Journal of Sedimentary Research. http://archives.datapages.com/data/sepm/journals/v55-58/data/058/058002/0247.htm Abstract: Gypsum crystals were grown in ...
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  • 22.2k
2 votes

is this a rose quartz

Yes, it is rose quartz. There are a number of mines in eastern New York near Vermont where rose quartz been found. Massive rose quartz can occur in granitic pegmatites. I could not google to find ...
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  • 5,871
2 votes

Sources or strategies for stone identification

Are your stones raw or are they cut & polished as they would appear if mounted in jewellery pieces? There are tests such as streak & hardness which can be done to raw stones but not to ...
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  • 20.7k
2 votes

When diamonds "migrate" from deep underground to the surface, do they maintain pressure inside when there is no more pressure outside? If so, how?

One of the more interesting examples of diamond maintaining high pressure in its lattice is discussed in this answer from Space Exploration SE. Put briefly, Ice VII inclusions have been found in ...
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