The Science of Gemstones

Ruby in its natural state

In Mogok, a seam of lava has pushed through limestone that was laced with specks of aluminium oxide, and a smattering of chromium. The lava’s heat melted the limestone into marble. The aluminium oxide in the marble was concentrated into corundum crystals tinged with distinctive chromium red. The Burma rubies known as “pigeon blood” are the finest in the world.

The world’s best sapphires are found in Kashmir – a “quintessentially rich, royal, velvety blue”. The crystals have formed where an underground pool of lava has cooled and solidified into granite, next to a deposit of limestone. Here, however, the geochemical furnace operates in a different way from the ruby mines of Burma: the gems are formed not in the marble but in the granite.

Just as iron-makers use limestone as a “flux” in a blast furnace, to remove impurities from the molten iron, so the limestone in Kashmir absorbs silicon from the molten lava. In pockets of the lava where the silicon is particularly scarce, the rock solidifies to corundum. Natural traces of titanium impart its blue colour.

In Montana, sapphires are created in the same way, where a six-foot wide “dyke” of lava has cut through limestone. According to one survey, the Yogo Gulch deposit contains 28 million carats of sapphire, making it among the largest deposits in the world. These stones are pale blue, and wonderfully transparent.

The six-pointed rays of a star sapphire

But not all sapphires and rubies are clear stones. Titanium dioxide (rutile) may form a form a network of tiny whiskers within the stone. Shine a light on such a stone and you see a six-rayed star.

Today, it’s not only Nature that creates gemstones. Small industrial diamonds can be made artificially; and it’s comparatively easy to manufacture ruby and sapphire. Artificial gemstones are not just created for the jewelry market: corundum is an ideal crystal to use in a laser. When you see the red ray of a laser in a supermarket checkout or CD player, you are seeing the amplified glow of a tiny gem of pure ruby.

More…

- how to make gemstones in the kitchen (well, with a bit of added pressure)

- pictures of famous gemstones

- just about everything you need to know about diamonds