When Betteredge describes first seeing the Moonstone diamond in Period 1, Chapter 9, Collins immediately presents the reader with powerful visual imagery of paralysis, depth, color, and intensity:

Lord bless us! it was a Diamond! As large, or nearly, as a plover’s egg! [...] When you looked down into the stone, you looked into a yellow deep that drew your eyes into it. [...] It seemed unfathomable; this jewel, that you could hold between your finger and thumb, seemed unfathomable as the heavens themselves. We set it in the sun, and then shut the light out of the room, and it shone awfully out of the depths of its own brightness, with a moony gleam, in the dark.

Betteredge is so stunned by even the "flash" of the diamond that Rachel gives him that he has to "bless" himself to express his surprise. All of the sensory language of sight that Collins uses here makes the diamond seem uncannily enormous and engaging. It's just a stone in the hand of a young girl at this point, but it's also an object that contains "a yellow deep" that the manservant finds difficult to describe. The imagery of its relative smallness—"you could hold it between finger and thumb"—is contrasted with descriptions of its enormous and supernatural profundity. It is "as unfathomable as the heavens themselves": so "unfathomable" indeed that Betteredge uses the word twice in the same phrase.

Collins also, even at this very early point of its presence in the story, gives a supernatural veil to the Moonstone. When Betteredge describes it later in the passage he says it does not merely reflect light, but actually, impossibly, "contains" it. The jewel is so valuable and so eerily beautiful that it shines by itself like a light is on within it. Its "moony glow" is still visible, coming from the "depths of its own brightness" as it scintillates  "awfully" alone in the dark. This imagery supports the narrative that the Moonstone is cursed. It isn't just a stone, even after it has been removed from the Moon God's Temple. It is an object with its own, frightening power.