Unlike stocks, bonds, and other investments, investing in jewelry is a guaranteed return. It is not a short term investment as some other financial instruments, but a long term one, as the value of vintage jewelry will continue to appreciate. It is a much safer long term investment, not only because value increases with age, but because the value does not depend on the market value of the material it is made from. In addition, fluctuations in the economy will also have limited impact, as those who are willing to purchase or invest in such antique rings are a niche market.
Buying antique rings will require some expertise and/or interest in the field. Just purchasing a ring based on current value alone will not be enough, and you might not see any meaningful appreciation unless you go for the right products.
For example, if there looks to be an increase in demand for art deco rings, then that would be a good time to add some of those pieces to your collection, or alternatively wait for the price to increase and sell some, maybe to recover your investment or switch to another form.
The biggest allure in buying antique jewelry as an investment is its low risk nature. As a safe, long term investment, nothing is better.