From Dwarf Fortress Wiki
Milk quartzes are unremarkable low-value gems that can be found just about anywhere.
 In Real Life
A whitish-grey, mostly opaque quartz crystal. Commonly found in many different places and conditions, indicating an unstable, "dirty" formation.
[INORGANIC:MILK QUARTZ] [USE_MATERIAL_TEMPLATE:STONE_TEMPLATE] [MATERIAL_VALUE:2][DISPLAY_COLOR:7:7:1][TILE:15][IS_GEM:milk quartz:milk quartzes:OVERWRITE_SOLID] [ENVIRONMENT:ALL_STONE:CLUSTER_SMALL:100] [SOLID_DENSITY:2650] Common to quartz gems [STATE_COLOR:ALL_SOLID:CREAM]