A ruby is a pink to blood-red colored gemstone, a variety of the mineral corundum. The color of a ruby is due to the element chromium. The brightest and most valuable shade of red called blood-red or pigeon blood, commands a large premium over other rubies of similar quality. All natural rubies have imperfections in them, including color impurities and inclusions of rutile needles known as “silk”. Gemologists use these needle inclusions found in natural rubies to distinguish them from synthetics.