You can buy something simple costume jewelry pieces for engagement. All online jewelry stores have a quality of excellent personal service representatives to customers who are willing to help you with your purchase. They can provide more information such as cut, clarity, color, and size as well as help you with all the options that might be available. If you are looking to buy diamond ring online then you can search for various online sources.
They can give you more information on return policies and insurance that is available, too. Just like a physical jewelry store you walk in, representatives of customer service are well trained and respectful.
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When purchasing a ring from an online jewelry store, you will know the size beforehand. There is no way for the jeweler to measure your finger, so the best thing to do would be measured inside a ring that you already have with a piece of string and then measure the string with a ruler, or next time you chance upon a jewelry store, have measured your finger.
Most online jewelry stores have to size the available ring before you ship your ring, but you know what size you need. Be careful if you are considering buying a ring that is heavily discounted.