Sand is not a material as such, but is a grain size on the scale of geologic materials. The scale goes essentially like this...clay,silt, sand, gravel, cobble, boulder.
Any rock/mineral that is reduced to this size is "sand". They come mostly from rocks and shells.
Silicates form the most common sands.
Feldspars and micas will continue to get smaller and form silts and even finer clays. In all cases, they are mostly the product of physical and chemical erosion...rock/shell particles being ground down by abrasion, collision, etc., or contact with acids (acid rain) and so on.