The piece they put in their eye to look at the jewelry. What

is that called and what does it do exactly?

Answer:
It is called a “loupe” and it is a magnifier. There are lots of important attributes that separate it from a normal magnifier, however. First, if the jeweler or gemologist is serious about his trade it will be a high-quality instrument. It will transmit light efficiently and offer a crisp image of the item that he’s examining. In addition, it will be corrected for spherical aberration. This means that a straight line will look straight, and not wildly curved like when using a kid's magnifier. This is partly accomplished through the use of three separate lenses in the device, which designate it a “triplet” loupe. The loupe will also be achromatic, meaning it will be designed to render colors in an accurate manner. Color is critically important in colored stones, and plays a huge role in determining value. It will also have lenses that have received a high-tech coating for protection, to eliminate glare, and to aid in accurate color rendition. A good quality loupe of this type made by Bausch and Lomb is about $40. If you see one at a store, flea market, or hobby shop for $10 or $15 you can be assured that this isn’t the type of loupe that a competent jeweler or gemologist would use. Perhaps the ideal place to get a loupe like this for a reasonable price is on eBay. There are loupes manufactured in a cold-war era spy satellite facility which meet all of these criteria, and have excellent optics that are bright, crisp, and accurate for color. The brand name is “Belomo” and they are made in Belarus. I think they’re about $20. I am a gemologist by trade and while I own more costly loupes, this is my favorite one.
Answer:
LOUPE
Answer:
a loupe