This is same like ceramic tiles but have more strength and lessor water absorption in it. the current market demand and architects requirement for making homes more beautiful took GVT in limelight. it is coated with glaze and same way like ceramic digital printing is done on top of it as per the requirement.
In double charge tiles, if you had seen tile manufacturing process article you might understand more better, how they are manufactured. To produce a double charged tile during manufacturing at pressing machine assembly first a body powder / clay is filled in the mould and then on top of it combination of two powder/ mineral / pigment is filled in design shap and the tile is press again in pressing machine.