|Corundum comes in a variety of colors with one color singled out for a different gem name. The general gem name for corundum is sapphire and sapphire can be virtually any color, but if a sapphire is red and the colorant is chromium, then it is called ruby.
The names "Burma ruby" or "Siam ruby" are reserved for the best blood red varieties although there is no guarantee that a ruby comes from one of those localities simply because of its name. The name "pigeon blood" is also used to describe the best ruby color. Ruby has traditional had a higher value than all sapphires.
Color in ruby may form in layers and is not always uniform. Color zoning can be difficult to see if the ruby is cut properly as the clear bands may not be visible in a faceted stone due to internal reflections. Inclusions are common in most ruby, and help to identify it as natural rather than synthetic. Ruby has been grown in the laboratory that is chemically identical to the original and fakes are sometimes passed off even as rough. See the synthetic section of these pages to find more information on synthetics and how they are manufactured.
When rutile crystals are present they may produce the star or catseye effect. It is not unusual for the rough to have been heat treated to aid in clearing the stone or improving the color.
All other colors of corundum except that colored by chromium is called sapphire. The blue color of corundum is probably the best known , and the word has become synonymous with blue as in "sapphire blue". The best quality of blue sapphire is sometimes called "cornflower blue".
The blue colors arises from the trace elements iron and titanium. Since there is some titanium present, rutile is a possible secondary mineral and can produce the same asterism as it does in star or catseye ruby. Star sapphires are well known and highly collected. Catseyes are known but more rare.
Some blue sapphires suffer from a gray to gray-black coloring. This can sometimes be fixed by high temperature which may impart a permanent cleaner blue to the stone. Heat treating is not uncommon.
Yellow sapphires range from bright taxicab yellow to yellow-orange in color and may be quite spectacular. One special color of sapphire, the pinkish-orange variety has a special name, "Padparadscha", and may command prices up in the ruby range. It is far more rare. Sapphire also comes in pinks, violets, greens, and wine colors.
The violet colored stones get their coloration from vanadium, the yellow and green from lower iron content, and the mixture of iron and vanadium produces the pink-orange variety.
Compared to most of the world, jewelers in the United States tend to favor darker blue as the best color, while others prefer the mid-point blues with slightly higher clarity.
Sapphire is one of the "big three" precious stones ... ruby, emerald, and sapphire.