Any corundum without trace elements is colorless. Corundum with trace chemical impurities makes the gems we know. Chromium gives the red to rubies, sapphire has iron and titanium, you can get other colors using vanadium or different ratios. Gems are neat.
Types of corundum maybe but I think that’s a stretch. Sapphire is (usually) blue Al2O3. Ruby is red Al2O3. Transparent aluminum is Al2O27N5.
Watch faces are often colorless sapphire; I think industrially produced.
Any corundum without trace elements is colorless. Corundum with trace chemical impurities makes the gems we know. Chromium gives the red to rubies, sapphire has iron and titanium, you can get other colors using vanadium or different ratios. Gems are neat.
Ah so my watch branded the face as “sapphire” because it sounded more sellable than “corundum”?