Top 10 Famous Individual Gemstones with a Star (Asterism)Asterism is a special optical effect some gems can exhibit by forming a star, usually a 6-rayed star. The best stars are created by Sapphires and Rubies, and they are called Star Sapphires and Star Rubies, respectively. Only 3 out of 100 sapphires and rubies produce stars, but only 1 out 100 produce both good color and well defined stars. The biggest producers of Sapphires and Rubies are Myanmar and Sri Lanka.
Occasionally, other gems can also form a star but nothing compares to sapphires and rubies. Also, sapphires and rubies have a hardness of 9/10, which is the 2nd best after diamond (10/10).
What creates those stars? Inside the gem crystal there are needle-like rutile inclusions that got trapped in the gem during its formation. Rutile is a mineral composed primarily of titanium dioxide. Rutile needles are often called rutile silk.
To see the star, gems don't get faceted - they just have to be shaped and polished as opposed to faceted. Facets would ruin the star effect. Gems with stars are cut "en cabochon" (French for "in the manner of a cabochon"). The resulting cabochon shape is usually a convex (rounded) obverse with a flat reverse.
Gems with a star, or "Star" gems, are highly sought after and are generally more valuable than their normal counterparts.
The Top Ten
This gem is large and unique because it has stars on both sides of the crystal (usually there's a star only on one side). This almost flawless, golf-ball-sized sapphire once belonged to banker John Pierpont Morgan who donated it to the American Museum of Natural History.
Fun fact: The Star of India was mined in Sri Lanka. - Metal_Treasure
Is the "Treasure" in your username a reference to gemstones? - Alkadikce
It's a large ruby with a well-defined and well-centered star (in addition to its intense color). - Metal_Treasure
It's the world's largest double-star ruby. Rubies with a star are rare but rubies with a double-star are even rarer. The owners of this old gem are descendants of the rulers of the Indian Vijayanagara empire. - Metal_Treasure
Blue is the most loved color in jewelry and when a blue sapphire comes with a star, it's even better. And more expensive, unfortunately. - Metal_Treasure
This star is so well-defined that looks unreal. And that vivid purplish-red color is also incredible. - Metal_Treasure
It's a very large black sapphire that formed a star. Now, that's a really rare combination because, 1) black is a rare color for sapphires, and 2) stars are rare in general. Also, the center of the star is much bigger and brighter than normal star sapphires. This gem was mined in Australia. - Metal_Treasure
It isn't very large but it's one of the most perfect star rubies. - Metal_Treasure
It currently resides in the Smithsonian Institution (it was donated in 2004 by Jeffrey Bilgore). - Metal_Treasure
The famous banker John Pierpont Morgan owned it but donated it to the American Museum of Natural History. - Metal_Treasure
It's the largest blue star sapphire in the world but you see that color isn't very intense and the star isn't well defined. Now go back to the beginning of the list to appreciate the colors and the stars of the gems on the top spots. - Metal_Treasure