Fluorite      Blackdene Mine, Weardale, Durham, England.  I suspect that this is a rather old specimen...  8.5 x 8.2 x 3.7 cm with cubes to 3.2 cm on edge.  Minor chipping throughout but not in what most would call bad condition.  The big twins are doubles, with two sides showing the penetration.  I've seen these in the literature but do not remember what to call them.  I have not seen this in most contemporary material from anywhere.  Small, ugly etched calcite in center of piece.  Great clarity and surface have mixed luster with corners being clearer (much like "Carthage Corners" seem on Tennessee material).  The first $300 gets it.