Zircon

Although zircons are actually rarer than diamonds and have different physical and optical properties, jewelers used natural colorless zircons as diamond simulants for centuries. However, they can make beautiful gemstones in their own right. Pair of round brilliant-cut colorless zircons, 3.08 cts, 7 mm, Cambodia. © The Gem Trader. Used with permission. (Cropped to show detail).

Article: What’s the Difference Between Synthetic, Simulated, and Created Gemstones?

Credit: The Gem Trader