I recently found this rock on one of my morning walks:
It is not the first time I have seen this sort of thing, namely a crack in rock (in this case I think it is sandstone, but I’m pretty sure I have seen similar in igneous rocks) neatly filled in with a layer of quartz. Does anyone know how this may come about?
It’s a part of a quartz vein. In geologically active areas, veins can occur in igneous, sedimentary and metamorphic rocks. These veins are usually the result of hydrothermal fluids forcing their way into existing rock under great pressure from below. As their name suggests, they are very hot and therefore very fluid, carrying dissolved minerals such as silica (SiO2) with them. The high pressure forces existing cracks and planes of weakness apart, and sometimes also creates new fissures for the fluid to move into and so reduce the pressure. As the fluid cools, mineral deposition and crystallisation occur. As it cools further, the deposited material contracts, cracking slightly, and eventually leaving behind an intrusion of which your sample shows a part.
This type of vein is not uncommon. In the Barberton area, they often contain gold as well, this having also been carried by the same hydrothermal fluid activity as the quartz.
'Luthon64
Mefiante: I thought something like that might be the case. I didn’t think of high temperature fluids though, so I asked myself how on earth enough SiO2 could dissolve in water, but I suppose under very high temperatures it is of course another story.
Thanks for the information. Now I can show the rock to me science class and know what to say about it apart from “Isn’t it pretty?” 
I neglected to mention a few important aspects. The mineral deposition is usually neither instantaneous (or nearly so), nor the result of a single hydrothermal action. While the activity is short in geological terms, it can extend over several thousands of years and is often episodic. Also, the hydrothermal fluids flow through the cracks, fissures, faults, bedding planes and other discontinuities, so that mineralisation happens gradually through slowish deposition as the fluid is cooled by contact with the surrounding rock, in repeated cycles. The fluid itself tends to be water-rich and can reach temperatures as high as 800°C. However, it doesn’t boil in small part due to its high dissolved mineral content, but mostly due to the extreme pressure it is under.
(More.)
'Luthon64
Mefiante: Thanks for the information. I knew it had to be on Wikipedia, but it is difficult to know what to search for without knowing what exactly it is you are looking at.
I am of course a bit skeptical of anything you say seeing as you lack spiritual intelligence, unlike a certain correspondent of ours on LitNet…
And that’s just as it should be. After all, I don’t, unlike said correspondent, have a direct feed from The Eternal and Immutable Truth™. :o
'Luthon64
Quartz, or possibly calcite. Try the acid test and drop some vinegar on the vein. If it fizzes, the white stuff is calcite. Here’s a link:
http://www.windows2universe.org/earth/geology/min_calcite.html
Or you can test the hardness. Calcite is soft (3 on Mohs Scale of Hardness) whereas quartz is 7. If the sample can scratch glass (which has a hardness of 5.5), the mineral is quartz.