A friendship gift has a scale. Too big and it creates an obligation; too small and it disappears. Jewelry in the middle of that range is close to ideal — it lasts, it is personal, and it does not demand anything back.
Keep it simple and wearable. A friend will wear a small everyday piece far more than anything that needs an occasion.
How much is too much for a friend?
If the gift would make her feel she has to match it, it is too much. The point is that you noticed something about her, not the receipt. A well-chosen piece under the price of a nice dinner lands better than one that costs more.
What makes it feel personal?
The note. A private message she can open with the box — one that references something only the two of you would understand — turns an ordinary piece into the one she keeps.