Piezo crystals, also called piezoelectric crystals, are a special type of crystal formation with unique properties that help it respond easily to external stimuli. Most piezo crystals in use today are made from polycrystalline ceramics, which have better durability and flexibility than naturally occurring piezo crystals. These crystalline creations are used in circuits and timekeeping devices, among other systems.
When piezo crystals encounter a force, they respond by creating slight electrical charge. The charge is equal to the force applied--in other words, if a crystal is compressed one way, it will exhibit electric polarity in one direction; but when compressed the other way, the polarity will flip around to match the new pressure. This response to external stresses makes piezo crystals unique, according to Azomaterials, a materials research website.
The relationship between electrical charge and mechanical force works both ways for piezo crystals. When the crystal is placed in an electrical field or when a charge is applied to the crystal, it will respond by straining, compressing based on the direction from which the current is coming. These changes happen very quickly, which allows scientists to make piezo crystals vibrate when connected to an electrical source. This vibration is steady, and helps keep time for some circuits.
"Soft" refers to the way that the piezo crystals are doped, or combined with other compounds for additional effects. Soft crystals polarise easily--they change quickly based on slight external stimuli, without much encouragement. This makes them sensitive but also delicate.
"Hard" piezo crystals have rigid ferroelectric properties and do not respond as easily to external stimuli. While this makes them less sensitive, it also allows manufacturers to use the crystals in devices that have higher electrical or mechanical stresses. They are more durable than the soft-doped versions.