Yes, it's an insulator, called a "Glass Block". Some are worth a fair amount of money. This one not so much I suspect, don't know much about values though. Perhaps others with more knowledge will chime in.
looks like a CD-1000, too bad it looks cracked in half sadly, but still, an early piece of telegraph history. Should have a little value even being damaged however. Pretty neat piece
Even in two pieces, probably still has some reasonable value since it is all there...in 1995 (my newest insulator book) a good one in aqua listed for $500-600.