Rock Tumbler Grit for Dummies

rock tumbling machine

Once upon a time there was a journalism major who wrote for her college newspaper. In her free time, she picked up a few bucks writing for this cool website that paid her to publish how-to articles. The coolest thing was that even if she didn't know "how to," no one there knew, either; so she'd still get paid even for utter bull. That, we believe, is how Megan Shoop and her post "How to Make Your Own Rock Polishing Grit" made it onto the internet at eHow.com and, eventually, to OurPastimes.com.

Shoop took a rather unusual tack for her assignment: she made it up out of whole cloth. Oh, she had "references" that told her that rock polishing grit is usually man-made silicon carbide and comes in various coarseness grades. She also figured out from somewhere – a friend who took "rocks for jocks"? – that beach sand is mostly silicon dioxide, the fairly hard mineral quartz.

Megan's solution? Buy some big quartz crystals and hammer the bejeepers out of them. No kidding: "Smash the quartz with the hammer. It should bread [sic] into several large pieces." "Hammer at the quartz... continue hammering until the pieces resemble unground sea salt [sic]. Remove about a third of the quartz pieces. This is the coarse polishing grit." "Smash the quartz with the hammer until the remaining pieces look and feel like sand. Remove another third. This is the medium polishing grit..."

...and there was more. Shoop's rather qualitative size descriptions are at odds with the quantitative grit sizes described in her reference, but hey: "unground salt" and "sand" are close enough for liberal arts majors, right?

Never mind the problems with size, though, Megan has a problem a lot more pressing. According to Shoop,

"Rock polishing grit is made of silicon carbide, a substance harder than most rocks. It is much the same as silica sand, a sand used in sandblasting with a primary component of quartz..."

...which is utter bull. Silicon carbide is one of the hardest substances known to man, only slightly less hard than a diamond. It's about a 9.5 on the Mohs hardness scale, where quartz ranks a solid 7. In other words, Megan's substitute grit would be useful only on minerals that are softer than quartz and rocks that contain little or no quartz. Anything else would simply grind the powder to flour so quickly it would be worthless.
Perhaps that's why the people who developed polishing grit for lapidary tumblers had to know something about hardness. It's for sure the reason why Shoop is picking up another Dumbass of the Day award, her third.

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