There's always going to be value in buying preloved/second hand fine jewellery but some eras yield more value than others. Why? It's down commodity prices (that means gold, platinum and diamonds to you and I).
Around the time of the millennium from around 1995 until 2003 gold and other commodities were at an all time low. That meant that jewellers could use more of it. When gold/platinum prices are high they use less. That means that items made around the low gold/platinum price era tend to be heavier and therefore better value for money than later items. In fact Tiffany & Co discontinued many of their heavier items towards the end of the noughties as they became too expensive to produce.