Vibration of Quartz crystal What does it mean when we say quartz crystal vibrates.it is very strange to read that a particular solid vibrates with period of 10^(-6) seconds.does it mean something else when it is written quartz crystal vibrates.One thing which is in back of my mind is that they may be talking about electrons moving from one energy level to another and coming back.please help to eradicate the doubt
 A: Quartz crystal is a piezoelectric material.  It produces and accumulates electric charge when subjected to mechanical stress.  The piezoelectric effect works both ways.  Not only is electric charge produced when mechanical stress is applied, but also mechanical stress appears when electric charge is applied.
Mechanical stress, if applied in sufficient amount, is accompanied by mechanical strain, or deformation of the material under stress.  In quartz crystal, strain appears as a bending of the crystalline structure when electric charge is applied across the plane of a crystalline sheet.
An oscillator circuit composed of an amplifier and a resonator applies charge to the thin sheet of quartz crystal, and the crystal mechanically vibrates because of the reversible piezoelectric effect.  The crystal acts as the resonator in the circuit.  It vibrates at a desired frequency based on its thickness, shape, and the lattice plane from which the crystal is cut.  The vibrating crystal acts as a filter which eliminates all electric signals with the exception of the desired frequency.
The vibrating quartz crystal feeds back its frequency to the amplifier.  As long as electric charge is continuously applied, and temperature does not vary too much, the mechanical vibrations of the crystal resonator are regular and dependable.
A: You probably know wobbly jelly.

(animation from Wiggle wiggle by Alpika Singh)
When you hit the jelly, the whole body wobbles for a while at a certain frequency.
And because the jelly is a soft material it wobbles with quite a low frequency.
The frequency also depends on the size of the jelly.
The same phenomenon also occurs with solids, e.g. quartz crystals.
The important difference is, that quartz is much harder than jelly.
That's why a quartz crystal wobbles with a much higher frequency.

(image from YaClass - Grundlagen der Physik - Physikalische Größen und Einheiten) 
To understand where the high vibration frequency comes from, let's calculate an example.
We assume a quartz slab with a thickness of $1$ mm.
The fundamental vibration mode is characterized by
the thickness being half of the wavelength $\lambda$.
So we have a wavelength $\lambda=2$ mm.
The speed of sound in quartz is $c=5800$ m/s.
From this we can calculate the frequency
$f=\frac{c}{\lambda}=\frac{5800\text{ m/s}}{2\text{ mm}}=2.9\text{ MHz}$.
As explained already in @Ernie's answer, quartz is a piezoelectric substance.
By using the piezoelectric effect
you can convert the mechanical vibration to an electrical voltage, and vice versa.
