A) It means that there was a large, catastrophic basaltic eruption at this time -- probably coming from IndiaB) It means that this clay layer has been metamorphosed under very high pressuresC) It means that there was an asteroid or comet impact somewhere on Earth at this time -- the shocked quartz was ejected from the impact site and distributed around the globeD) It means that there was erosion of an ancient impact crater somewhere nearbyE) It means that there was a very strong earthquake at this time -- intense pressure release from the earthquake caused the shocked quartz
Quartz is found in many (acidic) igneous and metamorphic rocks. It's generally clear, but metal ions in the structure can cause it to gain colour. Quartz is not an ore, because it is mostly just SiO2. For the other questions, check the article for quartz on wikipedia.
1) The mineral quartz can be found practically everywhere. It is a common mineral in mostly all granitic rocks, as well as most sedimentary siliciclastic rocks. It can also occur as a trace mineral in rocks such as volcanic rocks, and in sedimentary carbonates. Additionally if a rock that contained quartz was exposed to metamorphic processes, the resulting metamorphic rock could also contain quartz (depending on the degree of metamorphism). Additionally, pegmatites are excellent sources of quartz. 2) In the grand scheme of things, quartz is pretty useless. These days it is used in many electrical systems, and things such as quartz clocks. Quartz can be used an abrasive (grit) used for grinding, since it is relatively hard, and it can be melted down to form glass. Additionally, the many varieties of quartz are often crafted into gemstones and used as cheap alternatives to more expensive gems. 3) Quartz can form in many different colors including red, blue, purple, white, brown, pink, green, yellow (and others). 4) The change in color is due to the incorporation of different elements or ions in the crystal structure during crystallization in the magma. For example the inclusion of iron and aluminum in quartz can give you a purple amethyst. 5) Quartz really isn't valuable at all. It is extrememly abundant on earth, and even if it is sold in gemstone quality, it doesn't fetch that high of a value. 6) Quartz is not an ore because it doesnt contain any valuable elements or metals (ores) that we would typically mine and use.
Quartz is found in many igneous rocks, granite would be an example, but quartz crystals (and large masses fo quartz) are found as vein filling where they have been deposited as hydrothermal deposits, oftyen associated with a nearby large granitic body. Quartz is also the principle constituent of many sands, and sandstones Aside from the decorative value of quartz, and its related gemstone varieties the major use of quartz (in the form of sand) is in building materials (e.g. as part of concrete) and as an abrasive. Pure quartz in a well formed crystal is colourless and clear, amorphous masses are milky white, many coloured varieties, with their own names are coloured by various impurities - Jasper is blood-red due to iron for example. Quartz is not particularly valuable - a good sized crystal, reasonably well formed can be bought for a few pounds/dollars/euro though good quality gemstone variants are obviously more expensive. An ore is, by definition a mineral that is an economic source of a metal, so quartz is not an ore, though it might be a waste (Gaunge) material in the mining of some metals - gold can sometimes be found in the same veins as quartz for example
