From what I understand, true 'Agates' (slanged as Aggies) were prized by players since they were more dense and not as fragile as glass. Higher density = better chance of knocking an opponents marble out of the ring. Plus they were prettier than most clays although labor intensive to make by hand.
Along came the industrial revolution and companies started to make marbles more efficiently out of glass, trying to simulate the look (of these prized Agate marbles) hence the names ___ Agate Co. etc.
I wasn't around back then so this is purely my opinion. Orange you glad you asked?