What makes a ruby red and a sapphire blue?

They are the same mineral. Both are corundum, crystalline aluminium oxide, which in pure form is colourless. A trace of chromium makes it red and it is called a ruby; traces of iron and titanium make it blue and it is called a sapphire - and corundum in any other colour, including yellow, green and the pinkish-orange padparadscha, is also called sapphire. Ruby is the only variety with its own name.

The line between pink sapphire and ruby is a matter of trade convention rather than mineralogy, and it moves - which matters commercially, because a stone called ruby is worth considerably more than the same stone called pink sapphire. Corundum is also the second hardest natural substance after diamond, which is why it is used as an abrasive under the name emery.

Nearly all of it is now made rather than dug up. The Verneuil process, developed around 1900, grows synthetic corundum cheaply, and synthetic sapphire is what watch faces, camera lens covers and scratch-resistant phone screens are made of. Gem-quality synthetics are chemically identical to natural stones and can only be distinguished by their internal growth patterns, which is a permanent problem for a trade whose prices depend on the difference.