Yantras are geometrical figures intended to represent the basic energies. Just as every elementary geometrical figure, a straight line, a square, a cross or a triangle etc., has a symbolic value corresponding to the basic notions, similarly, the Yantras symbolise one or the other power or energy.
In other words Vedic Yantras are the graphic representations of Mantras. Yantras can be drawn, engraved or painted on a variety of substances. The classical eight tantrik surfaces are gold, silver, copper, crystal, birch, bone, hide (which can include any type of paper), and Vishnu stone (shalagrama).