1) Tripod- the only way you could get a stable shot would be to raise the ISO rating, which would horrifically ruin the picture, and make it all fuzzy and distorted. If you use a tripod, and there’s no movement at all, you can keep the ISO to the lowest, and there’ll be no blur or fuzziness in your image.
Put camera on a tripod
Put ISO to lowest ISO

2) For a perfectly exposed shot, you should use a shutter speed of like 2-5″, a semi-long exposure like that is going to make the image brighter ( like it was taken in broad daylight)
And the image won’t be dark at all.
Put Shutter Speed ( the fraction ) to 2.5″

3) The aperture is the amount of light the lens can absorb, so as you’d guess, you should put that down to the lowest to, so the lens could absorb the most lens. You didn’t mention exactly which lens you had, but the lowest you could get on a lens is like f/1.4, but if you’re not using a prime lens, it could be as high as f/3.2, it depend on the lens you’re using for the jewelry shot.
Put the aperture to the lowest it can go for the shot.
( zooming makes the number creep up )

Hope You Get A Nice Shot,
Nathan Grammatico