Like most gemstones, rubies that show phenomena follow the 4Cs of color, clarity, cut, and carat weight. However, the 4Cs must be considered with the asterism, or star, effect.
Color – Asterated corundum qualifies as a star ruby as long as it is properly polished with a star, and has any amount of red in it. No red means it is a star sapphire . This means the ruby can have a dominant color of purple, and so long as it has a modifier of red, it is still called a star ruby. It can also be silvery or white with faint amounts of pink and still qualify as a star ruby, though the price will reflect this. In R2773 below there are faint amounts of red present in the edges of the stone, so it qualifies as a star ruby. This rule also means pink star sapphires do not exist by definition.
R2773 | play | right | “R2773”Cut – All star rubies (and star sapphires) will be cut into rounded forms. The curved surface is what allows for the star to be created. You can even have a star ruby in a sphere shape, though you should not expect it to be gem quality unless it is synthetic.
A well-centered star is also one of the most challenging cuts to make due to the 3 dimensional alignment of rutile needles . Beware of perfectly cut and colored star rubies lacking documentation and lab reports. They are most likely synthetic, especially at lower price-points.
Clarity – Lack of visible inclusions is what makes ruby gem material desirable. However, a very specific type of inclusion is what causes the star in corundum. Because the star needs rutile needles to appear, the best clarity possible in star corundum is semi-transparent. It’s important that there are not too many needles, otherwise the stone can be opaque, negatively impact the color, decrease durability, and other undesirable effects.
The common star ruby shown is an example of having too many rutile needles. The body color is very silvery, opaque, and the red is very purple. This is what natural, untreated ruby material costing less than $1 per carat looks like.
No two stars are the same, with the stars being as unique as the growth characteristics of the ruby. Sometimes stars are fainter, stronger, or off-centered. The individual arms can be uneven, of different lengths, missing, etc. The way the star moves along the surface of each ruby is unique too. Some move around very smoothly, some do not move much, jump around, disappear entirely from certain angles, and more. Each star really has its own personality in appearance and movement, and makes matching pairs extremely difficult.
An ideal star would be centered on the ruby, show strongly, move smoothly across the curved surface, reach across the whole top of the ruby, and have all 6 arms, all being of equal lengths.
Because of the extensive grading criteria for the ruby stars and color, the overall standards for quality are more relaxed. Most star rubies either show a better star but a more silvery/white color, or better transparency and color but a weaker star. Nature rarely strikes a good balance between the two, making the rare few rubies that do reach this quality some of the rarest gems in existence.
While unusual, having a natural, double star sapphire or ruby is possible in two ways. The first one is that the silk inclusions in the ruby are re-aligned by twinning in the stone, more plainly phrased as irregularities in the crystal structure due to how it grew. This creates multiple white stars on the same stone. The second possible way is to have hematite needles (less common than rutile in corundum) and rutile needles present at the same time. This forms stars of different colors, one white (rutile needles) and the other yellow (hematite needles). Occasionally they will form with one star overlapping the other, and make a 12-rayed star with both white and yellow arms.
Generally speaking, this is not probable with gem quality rubies. For a star to be visible, it requires a certain amount and type of inclusions. The more inclusions required to create certain phenomena, the lower the likelihood that the stone is high quality. Stars with hematite and rutile will most likely be opaque black-star sapphires, though double-star rubies with these inclusions are technically possible.
Like normal rubies, synthetic star rubies can be created. There are two types; one where synthetic rubies are diffused with stars, and the other has natural rubies diffused with stars. In both cases the new, synthetic rutile needles will be confined to the surface unlike natural stars. They are formed by rutile needles inside the ruby as shown below. Natural ruby material will be heated in this case too, though heating by itself is not an indication of a synthetic star since most rubies are heat-treated.
While only a 2D image above, many synthetic star rubies look like they have a white star painted on, along with round rings on the bottom of the base. Some companies do make higher-quality, translucent star rubies.
Synthetic rubies are unique in that any repolishing or chipping will remove the star or parts of it from the ruby. Just like the process of diffusing color, diffusing rutile needles work from the outside to the inside, meaning the treatment does not penetrate through the whole stone.
While technically not synthetic, natural rubies with enough rutile needles in them can be heated to make the rutile recrystallize and show a star. They share the same problems as synthetic stars with losing the star to possible chipping and repolishing. Some people also treat gem material with stars to show better color, though this is a painstaking process since the silk will dissolve at high temperatures.