Sand is the result of mechanical erosion, rocks transported by water and wind, torn into pieces and rounded by the movement. It is called sand if the diameter of the particles is in the range of 0.0625 mm (or 1/16 mm, or 62.5 µm) to 2 mm. This is at least the exact geologic definition of sand. The particles consist mainly of silica or quartz, because this mineral is very resistant against erosion, so it is the last which remains. However, they can consist virtually of any kind of rock
To mine sand the easiest thing is to dig a hole, an open cast mine, where sand is abundant in the areas where wind, rivers or lakes deposited enormous layers of sand and gravel. Probably it is necessary to sort the sediments, the result is silt, sand, gravel, and cobbles, which may be sold separately. Such quarries are abundant and we do not list them on showcaves.com. What we are interested in is the rather rare underground sort of sand mines. They are so rare, because there are two important reasons against such a thing. First sand is not stable, it colapses and thus an underground operation would be very dangerous. And second it is too expensive to do underground mining when the sand is easily available in open casts.
Actually there are important restrictions to both arguments. Sand is actually very stable, under the right circumstances, which includes pressure from above. It may also be solidified by minerals from the groundwater, which fill the gaps and glue the grains together. This rock is called sandstone. If the minerals are soft enough it is possible to grind the rock to sand. So the best rocks to mine are hard enough to be stable and soft enough to be mined and grinded.
And the economic reason works actually also in the other direction. Underground mining is expensive, but transport over long distances also. So if there is no subsurface deposit available, it may be cheaper to mine underground. This was even more important in older times, when transport was difficult and expensive. So actually most sand mines are historic mines.
Sand or loess or soft sandstone are easily mined, since they can be extracted with pick and shovel. The mining often started with a subaerial pit until it went underground following the best layer. The mining is generally done in the old room and pillar method, where parallel passages are cut into the rock, crossed by other parallel passages. The result is a huge maze with a ceiling supported by huge pillars. The trick is, to leave enough material so the ceiling is stable.