I have done some unusual things (for me) but the sand, gun powder (Oh My!) and Curtis Classic are the true monsters of creation.
I have wadded sparkly ribbons into the mold and cast which turn out real nice. Layered metal flake is always a fun one. That is a lot of work though. Lately, I have been playing with some new material which requires first casting, then stabilization. I will post pics after the pen is finished. Pot scrubbers are always fun. I also took colored copper wire in red, silver, and blue and wadded up sections of each to make a patriotic type pen. The balls of wire have to be fairly tight wads going all directions for best effect.
As for the gold flakes, I would do as stated here. a drop of glue, put the flake on...or smear glue and drop flakes on. Cure and add some glue over the top. repeat until desired depth is reached. Try to do it on a pen that has some thickness to the material after turning otherwise the material you wanted to show may be removed.