Lots of factors will contribute to the quality of photos.
- Lens
- Camera sensor size. I think 60D has an APS-C sensor, which will tend to be noisier than the "same" image shot with a full frame sensor.
- ISO. 320 is pretty low, but you should always strive to use the lowest possible ISO, especially if reducing noise is important to you. For a shot like this, where you are using a tripod and your subject is pretty stationary, try manually setting your camera to the lowest ISO and adjust aperture and shutter speed accordingly.
- "Glow" in an image can be caused by the shutter speed not being high enough to "freeze" the movement of the subject. Is it possible sometimes you move your hand slightly?
- Image quality settings in the camera
- Are you using the highest resolution?
- Are you using "Fine" or equivalent?
- Are you shooting in RAW or JPEG?
- Proper exposure of the shot. If your image exposure is low and you post-edit to bring up exposure and/or shadows, this will amplify the noise (grain) in your photo, especially in darker areas.
- Cropping, which drops the total image resolution. This may cause a perceptibly lower quality image when rendered at the same size as before a crop because now there's less image data.
- Certainly others!
If you are shooting in RAW, then the images directly out of the camera will tend to be surprisingly bad and likely need manual adjustments. For a long time I did not realize that JPEGs are automatically being adjusted by your camera, so they will almost always look better when compared to the same RAW photo. You will need to do post-editing to achieve great results with RAW, sometimes including de-noise (remove grain), de-haze (may help with "glow"), etc.
If I were you, I would try to rest the necklace on a stationary surface, which will take possible motion blur out of the equation, and allow you to lock in 100 ISO and get proper exposure by using a slower shutter.