I have recently started using coloured pencils.  A very useful web site is www.art-is-fun.com.  On this site is the example of the marble, which I used as the basis for the marble drawing.  I did not have all the colours suggested as I was using Derwent Coloursoft pencils instead of the authors preference for Prismacolor coloured pancils, which I believe are from the USA.    However, the Derwent pencils are excellent for layering and I was pleased with my first result.