Sapphire

Sapphire is gem-quality corundum (aluminum oxide) in any color other than red; red corundum is called ruby. While the name evokes blue, sapphire occurs in nearly every hue, with non-blue stones called "fancy" sapphires.

Blue color comes from traces of iron and titanium, while other colors arise from different trace elements. The rare pinkish-orange variety is called padparadscha and is highly prized.

With a Mohs hardness of 9 and excellent toughness, sapphire is one of the most durable gemstones, making it ideal for everyday jewelry such as engagement rings and a classic September birthstone.

Like ruby, sapphire forms in aluminum-rich, silica-poor environments. Many of the finest blues come from metamorphic rocks and from alkali basalts that carry corundum xenocrysts to the surface.

Classic deposits include Kashmir (velvety "cornflower" blue from metamorphic rock), Myanmar, and Sri Lanka, where sapphires weather into rich alluvial gem gravels. Basalt-related deposits occur in Australia, Thailand, and Montana (Yogo Gulch).

Because corundum is hard and chemically resistant, sapphires survive weathering and concentrate in stream gravels, which are mined worldwide. Slow cooling and the availability of trace elements like iron and titanium determine the final color.

Look for a hard (9), transparent to translucent crystal that scratches topaz and quartz. Sapphire shows distinct dichroism (two color tones when viewed from different angles) and frequently contains rutile silk, color zoning in straight or hexagonal bands, and crystal inclusions.

Blue is most common, but check for fancy colors. Star sapphires show a six-rayed asterism from intersecting rutile needles when cut as cabochons.

Look-alikes include blue spinel, tanzanite, iolite and blue topaz, all softer than sapphire. Tanzanite and iolite show stronger, different dichroic colors; spinel is singly refractive. Synthetic sapphire is common, so curved growth lines or gas bubbles suggest a lab origin.

Sapphire is primarily a gemstone used in rings, necklaces and watches, and it is a popular durable choice for engagement rings. It is the birthstone for September and a 45th-anniversary gem; a famous blue sapphire features in the British royal engagement ring.

Synthetic sapphire is widely used industrially for scratch-resistant watch crystals, smartphone camera covers, optical windows, and substrates in electronics, valued for hardness and transparency.

Metaphysically, sapphire is associated with wisdom, calm and focus, though such properties are not scientifically established.