In 1956, Swarovski, working with Christian Dior, created the Aurora Borealis crystal effect, where a pastel iridescent coating is applied to one side of the crystal, allowing Swarovski to produce its crystals in many different colours. Since then, the finish on vintage crystals has been almost impossible for imitators to duplicate even with modern procedures, although there have been many attempts. If you have your eye on vintage Swarovski, make sure you know you're getting the real thing.
Check the clarity under a microscope. Vintage Swarovski crystals should not have any interior bubbles, scratches or swirl marks.
Ask for the original Swarovski packaging. Dependent on size, the bulk packages will range from 10 gross to 1/2 gross, in well-branded, sealed envelopes. If the crystals are older than 1969, the package may say "K.S. & Co. Trade Mark." If older than 1949, the package may say "D.S.W. Trade Mark." Between 1970 and 1980, the box read "D.S. & Co." It wasn't until after 1980 that the company started including the word "Swarovski" on its packaging.
Look for uniformity. Whether you're looking at a piece of jewellery, or just at a handful of crystals, they should all be precisely cut with identical facets pointing up and meeting at the same point.
Let the crystal sparkle in the light. Swarovski vintage crystals were made with special glass compounds that allow them to look more brilliant than imitators and have more intricate cuts.