While there are different options to buy diamonds, it would be the best idea to buy small loose diamond. When buying a diamond at any local store, it would be wise to ask for a 10x magnifying glass and use that to check if there is any impurities in the stone. If you wish to buy small and loose diamonds repeatedly for any purpose, it would be actually wiser to purchase a 10x magnifier for yourself. These special magnifiers are called loupe and are available online and from most jewelry houses. The most advanced option to check a diamond is to use the stereoscopic binocular microscope which uses dark field lighting to enhance the view. However, those are costly and used in the labs or by the professionals with elaborate setting for diamond testing.
The most commonly available shapes for loose diamonds are baguette, heart shaped loose diamonds, emerald, marquise, radiant, oval, and round, princess and brilliant. Jewelers or goldsmiths generally purchase loose diamonds of different shapes and also sell those to the public. They purchase diamonds in bulk and use those stones in different jewelry they make. These stones are also embedded in watches, ladies handbags and even shoes. When diamonds are used in any jewelry or accessory, it becomes much more costly and exclusive, let alone the increased beauty and class.
There are different portals of where people can actually find loose diamonds of their choice. You can, in fact, search for loose diamonds using different criteria including shape, cut, weight, color, clarity and price. There are different retailers. If you wish to buy the diamond offline, who carry the whole catalogue of available diamonds from all the wholesalers. The retailers are given the wholesale price of those diamonds and then they add their profit on top of that before selling it to the customer. Depending on the retailer, the price difference for the same stone could be as high as $10,000, especially for larger stones. The online search engine would give you the price from different retailers for the same diamond so that you can choose wisely.
While buying from a retailer who is asking for the lowest price is an attractive option, it might not always be the safest option. You should buy from retailers with a reputation and even then, you should not trust their words and ask for the certificate of the diamond. GIA certificate is the most trusted around the world for that.