Fake diamond earrings are common, and there are many good reasons for this.
First, remember that the only time that a fake diamond, such as a cubic zirconia, will sort of become obvious is when it is significantly larger than a diamond would be. The reason for this is that most cubic zirconias are colorless and flawless, but colorless and flawless diamonds are extremely rare and expensive. So if you have a CZ that is larger than what someone would expect to see in a diamond (say something in the neighborhood of 3-4 carats or more), it will almost look like too perfect of a diamond and may be suspected to be a fake. As far as fake diamond earrings go, however, almost nobody wears 3 carat earrings because that would be huge and awkward-looking.
So because diamond earrings are typically smaller, such as between 1/4 and 3/4 carat, cubic zirconia earrings will not stand out at all, and will look just as beautiful as equivalent-sized diamonds.
The second reason that cubic zirconia earrings are so popular is cost. Diamond earrings can be anywhere from $100 up through $1,000s of dollars, but fake diamond earrings can be purchased for $50 or less despite being the same size as the diamond ones.
The third reason for the popularity of CZ earrings is that earrings are not as prominent as a necklace or ring might be, and often are partially hidden behind a woman’s hair. Fake diamond earrings will not be scrutinized as intently as a ring may be (but even so, remember that most untrained people will not be able to tell the difference anyway).
Finally, cubic zirconia earrings look good! Their flawless structure and colorless body sparkles and shines as brightly as the most perfect diamonds would. It’s kind of ironic when you consider that, if buying low quality diamonds (with color and inclusions), you would actually be paying more for a gem that doesn’t look as good. But again, it comes down to the individual’s personal choice.