Garnets and rubies both are gemstones, but rubies are rarer and more valuable than garnets. Garnets can be be found in a variety of colors; pyrope, almandine and andradite are the most common red garnets. Rubies, red corundums that contains aluminum oxide and chromium, generally are more valuable than red garnets, so it is important to be able to tell the difference between them when buying gemstones. The so-called "scratch test" can damage the stone, so a test using light is best.
Pick up the stone you wish to test and hold it very close to your eye, toward a source of light, such as a light bulb or lamp. Never look at the sun. Look through the stone. You will see reflections from the light source.
Tilt the stone at different angles and slowly roll it around. Watch the reflections in the stone as you do this. Continue to move the stone around until the reflections appear as a spectrum or rainbow.
Note the number of rainbows that you see reflected in the stone. If you see two rainbows, or spectrums, that do not contain yellow or green bands, the stone most likely is a ruby. Rubies absorb colors in the green and yellow bands of the spectrum and so do not reflect these colors. If you see only one rainbow and it contains all the colors, the stone probably is a garnet. Garnets reflect only very thin bands of yellow and green.
Take your gemstone to a jeweler for absolute certainty. A jeweler will use a device called a polariscope to measure the refraction index of light through the stone more accurately than you can with the naked eye.