Possible to grow optical calcite?
The crystal structure is trigonal, so has very low symmetry and high chance of twinning or other line defects forming during, say, Czochralski type growth methods.  My guess would be the good optical quality stuff sat and annealed in situ for a long time to clear most of the twins/dislocations out.  

Support for this comes from A.J. Gratz et al., Geochimica et Cosmochimica Acta 57 491-495 (1993) where they observed, with AFM, calcite growth occuring primarily on screw dislocations.