seal = signet, regarded as good as a signature. Now that writing is more common it has become an ornament. It was worn round the neck (Gen_38:18, Gen_38:25), or worn on the right hand (Jer_22:24). Compare also Hag_2:2, Hag_2:3.
cruel = inexorable, hard.
as the grave = as Sheol. See App-35.
the coals thereof = its flames.
which hath a most vehement flame. Hebrew "flames of Jah": shalhebeth-yah. Render this: For love is strong as death, The flames thereof are flames of fire, Jealously is inexorable as Sheol, The vehement flames of Jah.