Elliptic Cone:

> implicitplot3d( z^2=x^2+3*y^2, x=-3..3, y=-3..3, z=-3..3, grid=[9,9,9], orientation=[60,60], scaling=constrained, axes=boxed );

>

> plot3d( { sqrt(x^2+3*y^2), -sqrt(x^2+3*y^2) }, x=-3..3, y=-3..3, view=[-3..3,-3..3,-3..3], orientation=[60,60], scaling=constrained, axes=boxed );

>