What temperature does stone have to reach for it to turn into refractive glass?
What do you mean when you say 'refractive glass'?
The usual way for natural glasses to form is to cool lava (usually basaltic, but not nessecarily) so quickly that crystals do not have time to form. The temperature at which this happens is dependent on the lava composition, as this sets its melting point.
Sorry for not explaining very well. Is it possible to turn normal rock into glass using extreme heat? If so, at what temperature would it have to be?
Not as far as I know. The way I've described is the only way I know that natural glass forms. When intense heat is applied to rocks, they undergo metamorphosis, in which their mineral structure is altered (extreme pressure often, but not always present during metamorphisis too.)
Resulting rocks show a vareity of textures, depending on the specific properties of the original rock and the conditions under which the change occured, but a glassy texture is not one of them. The page I linked to has more detailed information if you are interested.
If temperatures continue to increase, rock will melt completely and eventually cool to form an igneous rock such as granite.
The majority of rocks are composed of a variety of minerals, each with a distinct melting point. When you raise the temperature of a rock certain minerals will melt first. This is the reverse of the process by which a magma solidifies. [One of the reasons there are so many kinds of igneous rock is that as the first minerals separate from the melt they change its composition.]
If a magma is cooled suddenly, as matthyaouw has noted, then it will yield a glassy composition.
Refractive glass, by which I think you may mean transparent glass, is made industrially by taking very pure quartz sand and melting it. This sand is generally taken from dunes rather than from sandstone.
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