Final answer:
The phrase in question is a metaphor, a figure of speech that implies the heart feels as heavy as a rock, used to convey a strong emotional burden.
Step-by-step explanation:
The phrase “My heart was a heavy rock in my chest” is an example of a metaphor. A metaphor is a figure of speech where a word or phrase is applied to an object or action to which it is not literally applicable. In this case, the student's heart isn't actually a heavy rock, but the metaphor conveys the feelings of weightiness and discomfort in an emotional situation. This type of language is commonly used in artistic expressions such as songs, films, and literature to illustrate the connection between moods and the heart. It helps readers and listeners visualize and feel the emotions that the writer or speaker is attempting to convey, which is that they are experiencing a heavy, possibly sad or overwhelming, emotion.