Classically white crowns often had a metal core that provided strength, they are called metal ceramic crowns. The disadvantage of this was that the metal could show through producing an unsightly grey line around the gum and the crown itself can look lifeless due to the lack of translucency.
There was a drive by dental material manufacturers to overcome this by producing a metal free crown. The challenge was to produce a crowns that would be strong enough to last in everyday function. They succeeded, and produced Emax and Zirconia crowns.