Rubies are valuable gemstones that usually have distinctive red color, although you can also find specimens with purple or pink hues. The red color comes from the element chromium. Red stones that masquerade as rubies are usually made from garnet or red silica glass. These imitations are more fragile than real rubies, which rate 9 on the Mohs scale of hardness, just below diamonds. It's this characteristic that primarily allows you to test whether a ruby is genuine.
Clean the stone with a damp cloth or toothbrush to remove any debris or marks. Dig your fingernail into the gemstone to see if you can leave a scratch. If you can't, gently scratch it with a penny, a piece of glass, a steel knife and finally, rough silicate sandpaper. If any of these items can scratch the stone, it isn't a genuine ruby. Only a diamond, a sapphire or another ruby can scratch a real ruby.
Draw a facet of the stone across the face of the penny. If it leaves a scratch mark, try scratching a piece of glass and finally a knife blade. The stone is probably a ruby if it can scratch all these items.
Rub the stone against a clean white porcelain tile. If it's a ruby, it won't leave any color or streak behind. The absence of a streak isn't definitive confirmation that the ruby is genuine, but the presence of color does confirm that it isn't real.
Soak the stone in lemon juice. If it's a real ruby, it will be unaffected, but if it's a glass composite, it will begin to cloud over in a matter of minutes. You can also use jeweler's cleaning compound for this test.