When looking for expensive jewelry, you want to go for Diamonds. I recently found out the most expensive piece of diamond jewelry is rare blue diamonds. I would say it is the most expensive piece of jewelry in the world. Some pieces of blue diamond jewelry have even being marked up to the value of 15 million USD plus. When you think about that is almost out of 97% of the population of the worlds reach for affordability, so hopefully your girlfriend will not mind.
If your looking for a cheaper option then feel free to check the range of Jewelry at Treasured Jewels. All of the range is of high quality and price is low because of working directly with the manufacturer. So far I have only added 21 Jewelry items to the store, so when I have a bit more time I want to add more to the range of say a total of 50 to 70 quality items. Most of the items are between $7 USD and $30 USD.
Some examples of the Jewelry above from Treasured Jewels.