A good friend of mine checks each morning on the Web for the final “Jeopardy” question from the TV game show. I don’t go to movies or follow sports, so I’m often at a loss when it comes to many quiz show questions. But recently I was in a position to answer the “Jeopardy” question because of my early training in geology.

The category of the question I got right was “to ‘dum’ it up.” That means, in Jeopardy-speak, that the answer will have the syllable “dum” in it. The clue mentioned that there is a substance a chemist would call aluminum oxide that’s sometimes used as an abrasive. How could it be named with “dum” in the word?

Aluminum oxide, or Al2O3, is well known to geologists. You likely know aluminum oxide with certain impurities in it as the gemstone sapphire. With somewhat different impurities, the gem is ruby. So if you find a deposit of the right kind of aluminum oxide in the back of beyond, your financial problems could be over.

But most aluminum oxide in the world isn’t gem quality. Instead it’s the mineral corundum. That was the answer to the “Jeopardy” question. I knew the answer because like all geology students and many a rock hound, I learned the names and properties of scores and scores of minerals (and a few gems) when I was young. Call it my misspent youth.

Like sapphire and ruby, corundum is very hard. On the scale geologists use to measure such things, it has a hardness value of nine. Some gemstones are eight on the hardness scale. Diamond – the hardest natural substance in the world – has a hardness value of 10.

Most sandpaper is made of small quartz grains. Quartz has a hardness of seven. That’s generally hard enough for smoothing down a bit of wood. Depending on its exact chemical composition, garnet is a bit harder than quartz, and in a good hardware store you’ll find garnet sandpaper. Corundum is harder still, making it an abrasive for tough jobs.

The Wall Street Journal recently reported that Apple is investing $700 million to give its new iPhone and smartwatches what are termed “sapphire screens.” The idea is that the screen of the phone won’t be scratched as it rattles around in your pocket or purse with your car keys, and the watch face won’t be scratched if you scrape it against a wall – even a brick wall.

Mineralogy to the rescue. But don’t ask what proportion of “Jeopardy” clues I can usually solve.