This process begins by duplicating the first steps of the more expensive Gilson Opal.  A porous, very colorful opal is easily formed.  Gilson goes to great trouble to fill the porosity with more silica, thus a pure opal.  Another company decided to take a shortcut and produce a product that is not 100% opal.  They take the porous material and fill the holes with resin (epoxy glue).  This is similar to stabilizing turquoise.  The result is a very pretty product that is inexpensive and easy to work, but not pure opal.  The figures I've heard are approximately 75% opal and 25% resin.