3.2.2  Cone Object
  Here's another example showing how to use a cone:
  cone {
    <0, 1, 0>, 0.3    // Center and radius of one end
    <1, 2, 3>, 1.0    // Center and radius of other end
    texture { T_Stone25 scale 4 }
  }
  The cone shape is defined by the center and radius of each end. In this
  example one end is at location <0,1,0> and has a radius of 0.3 while
  the other end is centered at <1,2,3> with a radius of 1. If we want the
  cone to come to a sharp point we must use radius=0. The solid end caps are
  parallel to each other and perpendicular to the cone axis. If we want an open
  cone with no end caps we have to add the keyword open after the
  2nd radius like this:
  cone {
    <0, 1, 0>, 0.3    // Center and radius of one end
    <1, 2, 3>, 1.0    // Center and radius of other end
    open              // Removes end caps
    texture { T_Stone25 scale 4 }
  }