I would like to know what "advertisements in mother-of-pearl letters upon the glass-covered back" means in the following sentences:
She glanced through the fly-specked windows of the most pretentious building in sight, the one place which welcomed strangers and determined their opinion of the charm and luxury of Gopher Prairie—the Minniemashie House. It was a tall lean shabby structure, three stories of yellow-streaked wood, the corners covered with sanded pine slabs purporting to symbolize stone. In the hotel office she could see a stretch of bare unclean floor, a line of rickety chairs with brass cuspidors between, a writing-desk with advertisements in mother-of-pearl letters upon the glass-covered back. The dining-room beyond was a jungle of stained table-cloths and catsup bottles.
Carol (a librarian at St. Paul Library) and Kennicott (a doctor at a small town called Gopher Prairie), got married and came to their house in Gopher Prairie, Kennicott's hometown, after their honeymoon trip. Even before they unpacked their belongings, Kennicott excused himself for an hour to see how the things were going at his office, leaving Carol alone. Lonely, Carol went outside for a walk, and looked around the buildings in the town. Then she spotted a hotel named Minniemashie House and looked inside.
In this part, I could not imagine how advertisements looked like. Were the advertisements made of real mother-of-pearl? Or were they just written on the glass so that the letters looked like mother-of-pearl? I just could not imagine this part.
I would very much appreciate your help. :)