I never worry about diets. The only carrots that interest me are the number of carats in a diamond. ~Mae West
Ah, jewelry. Decidedly one of my obsessions.
As with colors and design lines in your clothes, there are flattering ways to wear jewelry and…less flattering ways to wear jewelry. If you follow a few simple guidelines you can be pretty sure that you're going to look your very best.
Here are the two elements I use to create said guidlines-
First up is size- often people assume that their jewelry should be proportionate to their body size. Although this does come into play, it's actually not the most important part. More important is the size of your facial features in relation to your facial plane. If you have larger features in a smaller facial plane, the scale of your jewelry should be larger. If you have smaller features in relation to a larger facial plane, your jewelry should be smaller.
Next comes contrast- if you have lots of contrast in your coloring (black hair and pale skin and eyes, for example) then you can wear highly faceted stones in brighter colors, and shiny metals. If you have low contrast coloring (dark hair, dark skin and dark eyes) then you'll look best in stones in muted colors, faceted or cabochon (non-faceted) and 'brushed' or patterned metals.
So, let's try putting this together.
If you have larger features in relation to your facial plane- if you would say you have large eyes/nose/mouth in comparison to your face, then you'll do well with large size stones, and larger pieces of jewelry in general. If you also have high contrast coloring, stay with big, bright color stones and/or large pieces with shiny metals. If you have lower contrast coloring, stay with the same large pieces but with muted coloring and maybe brushed or patterned metals.
If you have small features in relation to your facial plane- your eyes nose and mouth seem small in relation to your face- then you'll look your best in smaller sized pieces. 'But-' you say- 'I'm a fuller figured/tall woman and small pieces just get lost on me!' Ok, so here's how to work with body size; if you're fuller figured, taller, bigger boned with smaller facial features and you wear larger size stones and larger jewelry pieces overall to work with your body size, it overwhelms your facial features and your facial features end up getting lost instead. A better approach is to group multiples; have a slightly larger piece with pave stones, perhaps, or several smaller scale pieces together. A few necklaces, or more than one pin, maybe. The object is to have the eye-catching points- the stones or stations or whatever- be similar scale to your features, and yet avoid having the jewelry get lost by having more pieces.
Of course there's a judgement call involved in determining what 'large' and 'small' mean for you. What's 'large' on a 4'8" size 2 woman will quite possibly be downright 'small' on a 6' 1" size 24 woman, right?
As with all my other suggestions, the idea is to simplify life. If you know your best colors, pick 4 or 5 to stick with, know what design lines work best with your proportions and pick a handful to use consistently, and know how best to wear your jewelry and other accessories, then life just gets easier, and it all becomes about figuring out new ways to express your aesthetic.