More than perhaps any other mineral, rutile has an affinity for growing as prism-shaped crystals within other minerals. Long prisms of rutile occur in many different gem minerals. Quartz, corundum (ruby and sapphire), garnet, and andalusite are some of the more familiar.

Sometimes these needles are coarse and clearly visible within the gem, as in many specimens of rutilated quartz. These needles produce attractive and interesting novelty gems when they have a pleasing color and arrangement. See the adjacent photo of rutilated quartz.

In some gems, such as ruby and sapphire, reflections of light from a network of fine rutile crystals within a properly cut cabochon will produce a beautiful “star” of light on the surface of the gem. Gem rubies and gem sapphires with this star are known in the trade as “phenomenal gems”, and the phenomenon of the star is known as “asterism”. See the adjacent photo of a light blue star sapphire named “The Star of India”.

In other gems, one direction of parallel crystals will form a line of light on the surface of the gem known as a “cat’s-eye”. The phenomenon that produces a cat’s-eye is known as “chatoyance”, and gems that exhibit that phenomenon are said to be “chatoyant”. The best-known gem for its chatoyance is cat’s-eye chrysoberyl.

Physical Properties of Rutile
Chemical Classification	Oxide
Color	Red to reddish brown, black, yellow to gold
Streak	Red to brown
Luster	Adamantine to submetallic
Diaphaneity	Opaque, transparent on thin edges
Cleavage	Good
Mohs Hardness	6 to 6.5
Specific Gravity	4.2 to 4.4
Diagnostic Properties	Luster, color, specific gravity, prismatic crystal habit
Chemical Composition	Titanium oxide, TiO2
Crystal System	Tetragonal
Uses	An ore of titanium, pigments, inert coating on welding rods