This is the jewel for a vintage chronograph’s center second hand under approximately 60x-to-100x magnification on a very high quality microscope with fiber optic halogen lighting. As you can see this jewel is bad and has many cracks causing the chronograph to not function properly. What is of note here is that these cracks are not visible to a watchmaker looking though a 5x or 10x loupe. In-fact even a medium quality microscope with average lighting under high levels of magnification did not show these cracks as the light did not pass-through adequately. The problem is getting the light into the depth of the jewel to see the occlusions— they are deep into the jewel and required higher quality optics and lighting to see— while these cracks look evident in this picture, they were in-fact strangely hidden. Most jewel cracks are not like this— you can see them readily under 5x or 10x but some will more difficult to spot without high quality optics and lighting. Better lighting may compensate for less magnification, but in the end you benefit from good setup to see this.
It’s difficult to know what it takes to see something, everyone’s eyes are different and the impact of light and visible depth for a specific optics setup. Optics for a microscope can vary considerably depending on quality and the specific lens design/setup . I looked back at this jewel using lesser magnification and tilting it under the light while knowing what I knew from the better setup, I began to spot some (not all) of the cracks; however, I might have missed them if I didn’t know they were there already. I will say this— a quality halogen fiber optic ring light in a microscope makes a big difference compared to the cheap led ring lights being sold today. So a microscope with the better lighting but lower magnification maybe could spot this, I don’t know. The Swiss brand accounts require at least 60x from watchmakers. Also the magnification in this picture is a function of the lenses on my microscope (Barlow, eyepieces) and whatever digital zoom the usb WiFi microscope adapter is adding. So I give an approximate range. I would say it's possible one can see some cracks with lesser magnification with fiber optic lighting but you can’t see the full extent until you get to about 60x. I tend to agree with the Swiss that 60x should be the minimum goal for catching most issues but would add fiber optic ring lighting is by far the best lighting approach from my perspective.