Sand is made of quartz, which is transparent. Sand doesn’t look transparent because it is not polished. [This is how glass looks after tumbling around the ocean for a few years](https://www.youtube.com/watch?v=ICCpObJTt0M) And [This is how sand looks under high magnification](http://www.geologyin.com/2018/01/this-is-how-sand-looks-magnified-up-to.html) (the first two photos are from a coral reef, not common sand). They aren’t that much different. In addition to being unpolished, some of the sand grains are cloudy- most quartz is [somewhat cloudy, like this](https://elementsofrejuvenation.com/products/natural-arkansas-milky-quartz-crystal-point) The cloudiness is water trapped during the crystallization process, it would boil away if you fused it into glass.
With all that said, you don’t really make glass out of sand. You make glass from *extremely pure sand*, plus about 30% limestone and salt. The salt is critical, as it melts, attracts impurities, and doesn’t mix into the glass. It is possible to melt quartz into glass, but it requires extremely high temperatures, and it makes very brittle glass. The main impurity is iron. Iron oxide (rust) gives the yellow- brown color you see on many beaches. In glass, it loses the oxygen and gives a faint green color. You can see that green color if you look at the side of window glass.
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