0 Members and 1 Guest are viewing this topic.
It is a very long story (millions of years to be exact) but I'll try to cut it short. That's OK.. I got time !!It starts with a volcanic eruption (or sometimes within the magma without an eruption) ooooh !!..wooosh wooosh !!..YAYYYYYYY !!!. The molten lava solidifies,Just like wifyes sponge cake !! and the volatiles leave bubbles, which are not filled up as the viscosity of the lava increases dramatically on cooling. So here we are with something like cheese with holes in it (also created by volatiles, in this case by products of the fermentation process). This is wonderful thank you Eric Next, it starts to rain. Me luffs the rain...it's well....rainy ! The water dissolves all kinds of minerals, like SiO2 or Al2O3, and this water accumulates in the holes. The minerals start to crystallize and form nodules, also known as "geodes". In the case of SiO2 you will obtain agate, with Al2O3 you obtain corundum. I think we have what must be Geodes too...my son has a whole big bunch of themNow of course, the water has dissolved other things besides 2 or Al2O3, and this "impurities" are responsible for the colours. THANKYOU Because the concentration of this "impurities" is not constant, you can obtain striped varieties, and so on.Aha !!..this explains it...MarvellousWether it will be a gem will depend on the colour, sometimes on the stripe pattern, and on transparency. If your stone is opaque rather than transparent, it may be just as useful for technical applications as the gem. The "rubies" is a watch for example are are often opaque corundum rather than the transparent red variety.To be complete : diamonds are not formed by carbon dissolved in water crystallizing in the bubbles, bu by the pressure on the carbon particles as the volcanic rock solidifies and shrinks. And occasionally, some minerals that are neither the agate nor corundum type qualify as gemstones (e.g. malachite, which is actually a copper mineral or lapis lazuli, which is actually a mixture of different minerals). THANKYOU ..Yes..I was upset to not see a diamond amongst my collection !!..I wonder why that is !! ?  
Good work Eric. I do have a couple of clarifications. For example Sodalite is a feldspathoid or a mineral resembling feldspar and occurring in rare igneous rocks. Amethyst is a purple quartz most often found in Pegmatite dikes, as is the rose quartz, Amazonite is a green microcline feldspar found in granites; Aventurine can be the name given to either a type of feldspar or a type of quartz. An interesting thing about all of the tiger eye gemstones is that they are all quartz replacement of different types of amphiboles, which are fibrous silicates.
Next, it starts to rain. The water dissolves all kinds of minerals, like SiO2 or Al2O3, and this water accumulates in the holes. The minerals start to crystallize and form nodules, also known as "geodes". In the case of SiO2 you will obtain agate, with Al2O3 you obtain corundum.
Perhaps but not very likely. The stone age peoples had to be VERY observant of their environment as their lives depended on it. I doubt they missed much as all of these stones have been valued for well over 2000 years