Answer:
Hi! This sentence is a metaphor to express that flowers were so beautiful and vibrant that they seemed to be jewels gleaming under the sun.
Step-by-step explanation:
Let's remember that metaphors are used to make comparisons; a metaphor is a figure of speech in which a word or phrase is applied to an object to which it is not literally applicable. In this case, for example, flowers are not capable of gleaming, but the comparison with the shining jewels under the sun is made in order to express how beautiful, vibrant and shining they were.