The word electricity comes from the Greek name for amber. Amber gets its name from the Greek word for electricity. Amber like plastic is an amorphous substance. Amorphous means with out structure. All the molocules are lumped together with no special order and when you rubbed the stone a static charge was developed.
Static electricity is the build up of negatively or positively charged particles.
Molocules are made up of atoms, which have electrons spinning around a positive center called the nucleus. Moving the stone back and forth on the cloth actually rubs electrons off the cloth and onto the stone. The side of the stone that was rubbed has a build-up of electrons, making that side negatively charged. The flour is also made up of atoms. Holding the negatively charged amber near the flour causes the positive part of the atoms in the flour to be attracted to the stone.