“bribe.” The Hebrew is shachad (#07810 שֹׁ֫חַד), and it has two meanings, “gift” and “bribe.” In this context it is clearly a bribe.
“magic stone.” The Hebrew reads “a stone of favor,” i.e., a stone that brings the favor, or grace, of the one to whom it is presented. The Hebrew word chen (#02580 חֵן) is favor, agreeableness; or charm and grace in the sense of pleasant, agreeable qualities, as we speak of someone being charming and having social grace. The Hebrew is hard to translate. A very literal reading of the stanza would be, “A bribe is a stone of favor to its owner.” In other words, the owner of a bribe is overconfident, and believes that his bribe will work the way he intends it to, which sadly, much of the time, is true. Because the person who uses bribes thinks they work all the time, “like magic,” the translation “magic stone” seems to capture the sense of the Hebrew text and some modern translations use that phrase (HCSB; ESV; NRSV; RSV). Another common translation is “charm,” but saying a bribe is a “charm” to its owner did not seem to carry the sense of the test as clearly as “magic stone.”