Ruby and sapphire are the same mineral. Both are corundum, aluminium oxide, and the only difference is colour: corundum coloured red by chromium is called ruby, and corundum of any other colour is called sapphire, qualified by that colour. There is no mineralogical boundary, and where exactly pinkish-red stops being ruby and starts being pink sapphire is a trade convention that differs between markets, with American practice historically stricter than Asian.
Corundum is 9 on the Mohs scale, second only to diamond among natural materials in common use, and it is tough as well as hard, with no cleavage. This makes it an excellent stone for wear and also an industrial abrasive; emery is impure corundum.
The chromium that reddens ruby also makes it fluoresce under ultraviolet, so fine ruby glows in sunlight and appears to burn from within. The same property put ruby at the centre of twentieth-century physics: the first working laser, built by Theodore Maiman in 1960, used a synthetic ruby rod.
Synthesis matters commercially. Auguste Verneuil's flame-fusion process, published in 1902, produced synthetic ruby in quantity and cheaply. Synthetic and natural ruby are chemically and structurally identical, distinguishable only by growth features. A very large proportion of red stones in older costume and even fine jewellery is Verneuil material, and much natural ruby on the market is heat-treated or has had fractures filled with lead glass, a treatment that is disclosed inconsistently.
The classic source is Mogok in Burma; Mozambique is now a major producer.