Opaque

The opposite of transparent, an opaque gem is  not able to transmit light, therefore you cannot  see through it.

Most  opaque  gems  are  cabochon  cut.  A  halfway house between opaque and transparent  is  translucent.  The  best  way  to  explain  the    difference  is  by  comparing  transparent  gems  to glass, translucent ones to stained glass and  opaque gems to a wall.  If you were to put a  large gemstone in front of this text and could  still read it, then the gem would be transparent.  However, if you could recognise that there was  text on the page but couldn’t read it, then the  gem would be translucent.