Because the poem comes to us in translation, we lose some of the nuance and poetry of reading it in its original form. However, figures of speech do shine through. To begin with, the river is personified or given human characteristics: when we first encounter it, it "frowned" like a human would, which indicates it may be an unfriendly force. Later, when the disciple's costly bangle falls into the river, we are told the river "stole" it, as if this were the deliberate act of a human being.
The poem also uses imagery, which means description using the five senses. We can see the wooded hills around the river and the bejeweled gold bangles set with diamonds brought by the rich disciple. We can picture the teacher calmly reading his book by the river as the disciple desperately searches the river for his bangle.
This poem also uses paradox to make its point. When, as the sun is setting, the disciple begs his teacher to find his bangle for him, the teacher does the opposite of what the disciple expects. The teacher, instead of going into the river and helping him recover the costly piece of jewelry, throws the other bangle in the river and tells him the first one is where the second one is: that is to say, now lost. This invites us to question how being "lost" is a matter of perspective (the personified river may "know" where the bangles are), and, more importantly, to question what is valuable: clearly the teacher values a costly material ornament differently than his student does.
I think that one can examine the imagery used to describe the bangles the first time the guru examines them when given by the student. An example of personification could be seen when the diamonds "darted shafts of light." The implication here is that the diamonds have come to life, reflecting the severe love the student holds for the bangles, almost to value them more than anything else. The personification reflects how things, according to the student, hold value and meaning. They seem to hold their own meaning, being their own agent of action, above all else. This might be why the very next line reflects the loss of this, when Tagore writes that "it slipped from his hand and rolled down the bank." Again, personification might be present in describing the student's love for the object. In these figures of speech, the idea of the object holding value is something that brings out how the student viewed the objct, explaining why he was destined to lose it.