Buying a pre-owned Tiffany & Co. engagement ring can be the best thing you ever do!Diamonds and platinum are two of the most durable substances known to man. Diamonds are already around 4 billion years old when mined - so what's another 5 or 10 years on top of that?A second hand ring can look as good as new and save you upwards of 25% of the original price.If you are considering buying a pre-owned Tiffany & Co. engagement ring here is a simple guide as to what you should look for.1. Good photography. Without good quality, detailed and clear photographs, how will you know what you're looking at?!2. No to collection only!Do you really want to pick something up from someone's house, who you don't know, with a wad of cash in your pocket? The answer is a definite 'no'!3. Stamps and hallmarks. Every ring should be stamped Tiffany & Co. Pt950 or Tiffany & Co. 750 Pt 950. Occasionally a very old ring might be stamped Tiffany & Co Pt900 IRID or just T & Co. The signature might be worn and depen...