Alexandrite Doublet, Ruby Doublet, Sapphire Doublet are the main doublets manufactured due to the cost of the respective Natural Gemstones. A doublet can be made from Natural gemstone material (bottom) with a crystal or quartz table (top),  the top is usually man-made. Or a doublet can be made from only lab created material. A doublet can also be made from only simulated materials. This should always be disclosed in the items listing, so you know what your paying for. A natural doublet will command a higher price than a lab created and a lab created will command a higher price than a totally simulated doublet. If it is not disclosed in the listing it is best to avoid that listing and not buy from the seller. Always look for the word Natural, when purchasing a  faceted gemstone. If you don't see it, than it's best to assume the gemstone to be Lab created (identical to the natural- just not from the earth) or simulated (could be any substance, usually glass) and make your purchase elsewhere. Following this simple rule can help you avoid being taken advantage of and wasting your hard earned dollars to less than honest dealers.