Bausch + Lomb has a wonderful reputation for top-of-the-line loupes.
There are also high-quality loupes made by Nikon and Zeiss but they can exceed $100. I don’t know that you need to spend that kind of money.
Bausch + Lomb offers a very fine, optically-correct gemological loupe for about $50 which is, in my opinion, the best value for the money. Because it has a smaller diameter in the loupe area, some people complain that you have to move it around to take everything in. But you’re looking at – or for – microscopic things inside or on the surface of a stone. A smaller diameter is actually better for gemological purposes because it helps you focus more critically and precisely.
At the most affordable end, the BelOMO triplet loupe is worth considering as well. Like the Bausch & Lomb, it has a reputation for accuracy and retails for less than $50. BelOMO is made of steel and is a bit larger and heavier than the tiny B&L. This loupe possible suited more for coins.