A sapphire is a blue corundum gemstone. "Sapphire is the most precious of blue gemstones," the website minerals.net says. "It is a most desirable gem due to its colour, hardness, durability, and lustre." Because of the gem's desirability, some try to pass off a fake sapphire made of glass as the genuine article. With a sharp eye, though, you can tell the difference between the real thing and a fake.
Place the sapphire under a jeweller's loupe or magnifying glass. Look for tiny bubbles in the blue stone, which indicate the sapphire is a fake made of glass.
Keep the sapphire underneath the magnifying glass or jeweller's loupe. Look for tiny scratches on the stone. Tiny scratches are telltale signs of glass.
Take the sapphire out from under the magnifying glass or jeweller's loupe and examine the facets. If they look simple or rounded you most likely have a fake. A real sapphire has "sharp, crisp cuts," according to TheDiamond.com
Turn the sapphire upside down and look at the setting. A bezel setting, with a band of metal surrounding the stone, or a closed-back setting are commonly used with fake stones.