Background:
It is important
to understand that Abrasive jets are not the same
thing as Water jets, although they are nearly the
same. Water jet technology has been around since
the early 1970s or so, and abrasive jets extended
the concept about 10 years later.
Both
technologies use extremely high pressures, and allow
the water to escape through a very small opening.
Water jets use the beam of water exiting the orifice
(or jewel) to cut soft stuff like rubber, paper,
and cardboard, but are not effective for cutting
harder materials.
Abrasive jets use that same beam
of water to accelerate abrasive particles to speeds
fast enough to cut through much harder materials.

(Graphic courtesy of OMAX Corporation)