If you attend the shopping spiels....or any of the diamond/colored gemstones talks....they definitely imply that buying from <insert hocked jewelry store here> is a "wise investment".
People on these boards tell stories about how they paid $xx for a piece of jewelry then got an insurance appraisal for 2* $xx so it was a "great buy". Even that implies that they think that somehow they will be able to re-coop that original 'investment' at some later time.
Earlier Orator mentioned how you will find the 'real' value of your piece when you try to sell it. Go to Craigslist/eBay and see what old engagement rings are selling for (not listing, but selling). It is shocking the discounts you can find!