Shakespeare's diction in "Sonnet 55" may be considered harsh when describing the realities of this world and lofty when talking about his love. The theme of the poem is that while the base things of this earth will pass away, the beauty of this woman will remain forever pure and bright within the lines of this sonnet.
The diction (word choice) to describe the harshness of the real world includes the following:
"marble"--a beautiful but hard stone
"gilded"--looks like gold but only gold-covered
"unswept stone besmear'd with sluttish time" - hard and unyielding rock, though long-lasting, is marked with the dirt and grime which accumulate over time.
Then begins the more violent imagery of a war waged on this earth by time. There are "wasteful wars" which knock down the once-beautiful statues and monuments and "broils" the mortar out from between stones--undoubtedly causing structures to fall. Mars, the Greek god of war, can destroy everything with his sword or by burning--everything but her memory, even in the face of death and enmity, can and will be destroyed. The ravages of time will "wear this world out to the ending doom" when everything will be destroyed.
The sonnet begins with an elevated diction of praise, as noted in words such as "powerful," "shine," and "bright." Then the words an images, as noted above, turn harsh. The shift in tone to a more lofty diction happens again primarily in the final five lines of the sonnet. Words like "praise" and "arise" and "lover's eyes" (a traditional image of beauty) depict an entirely different and softer tone. Even the words "live" and "dwell" and "posterity" are a stark contrast to the grimy, finite, "doomed" world.
This use of contrasting images and words--one to reflect the beauty and greatness of love or a loved one who will liveforever for posterity through poetry and one to depict the starkness and finality of a fallen world--is not uncommon in Shakespeare's sonnets. I've included a great enotes link below for further study.