This is a Raw Fluorescent Calcite from Santa Eulalia Mexico. S.W. Green.  It measures approx. 147 x 56 mm: thickness is approx. 20 mm: weight is 196 grams. 
 
Fluorescent Calcite is a mineral that commonly fluoresces with exposed to UV Black Light. It fluoresces better with short wave than long wave. Calcite itself comes in a variety of colors such as White, Black, Green, Yellow, Orange and Red. It is found worldwide and has a hardness of 3.