Those are definitely the best of the best! I live in Italy, and the better mercerie (fabric and thread shops) offer similar buttons which they call "curvÃ¨ madre perla". The best are the thicker ones, and the finishing is usually immaculate as are yours.
Lots of middle range Italian shirtmakers use plastic buttons on their shirts, and oftentimes insist that they are mother of pearl. I used to "educate" them that the best way to tell oher than a visual inspection was to place the back of your hands upon the button's surface... If it is warm, it is plastic, if it's cool, then it's mother of pearl. You can alsot tap the button on a front tooth (yours!); there's a big difference in the vibrations of the two buttons.
If the clerk is still insisting, take a pin, heat the tip with a lighter, and touch the underside of the button. It will stick into the button, and produce a slight plume of smoke and the distinctive plastic odor!
The MOP buttons can make a mediocre shirt shine.