The diamond Moonstone is from India, and the main characters suspect throughout that the three Indians who appeared at the Verinders' country house are somehow involved in the diamond's disappearance. Franklin Blake was educated in a foreign country, and Betteredge doesn't trust the "foreign sides" to his character. Ezra Jennings, the doctor's assistant, is only half English – and so many characters do not trust him. Being non-English, in the world of The Moonstone, is enough to make you a suspicious character.
Although both Mr. Murthwaite and Godfrey Ablewhite are able to disguise themselves effectively as Indian, none of the Indian characters are able to disguise themselves effectively as English. To be non-English, in the world of The Moonstone, seems to be an indelible mark on one's person.
Betteredge personifies the Moonstone when he says that their "English house" had been "suddenly invaded by a devilish Indian Diamond" (188.8.131.52), suggesting that the diamond itself represents the invasion of foreignness into England.