When John ascends in the Spirit through the door in the sky, he sees a throne and on the throne “sitting” (Gr. kathemenos ). The Greek doesn’t have the article; in 4:2, it is not “the One who sits” (though it is in 4:3), but simply the participle. Enthronement is not accidental, but of the essence. What John sees is Enthronement itself enthroned.
The Enthronement looks like a jasper and sardius, surrounded by a rainbow like emerald (v. 3). This is the Rock of Israel, but not a rough desert rock of granite or quartz. Here is the Rock fo Israel depicted as a precious stone.