Clear Quartz
Rock crystal was widely believed in classical and medieval Europe to be water frozen so hard it could not thaw; the Greek krystallos means ice. The idea persisted into the seventeenth century.
Quartz spheres served as burning glasses and as scrying tools, sometimes in the same household, and the two uses are harder to separate than later accounts suggest.
Quartz is piezoelectric: squeeze it and it generates a voltage, apply a voltage and it flexes at a precise frequency. That property keeps time in watches and sets the clock in essentially every computer, which makes it the one stone on this list with an unambiguous modern job.