You can't catch a rainbow, 
However hard you try;
Splashed across the sky,
It is an ephemeral show.

The colours shine and glow,
Pretty dazzling to the eye;
You can't catch a rainbow,
However hard you try.

A symbol of hope it is though, 
Peeping from the clouds high,
It is beautiful, you can't deny;
A heavenly sight from below,
You can't catch a rainbow.

The above form is a Rondel.

It is another French form and the rules are as follows:

Poem consists of 13 lines in 3 stanzasRhyme scheme: ABba/abAB/abbaA (uppercase letters are refrains)Usually 8 syllables per line.

(I have not paid much attention to the syllable count)