HPA 60 Garnet For Step 1 Tumbling

Hmm, I'm not really sure of the conversion either, but I'm thinking you have the equivalent size as 80 grit (based on the 80 screen). Some people use straight 80 SiC for coarse grit, but most use 60/90 because it is cheaper than a straight size grit.

Using your garnet grit won't hurt anything, but I see two reasons why it won't be as effective as SiC on quartz hardness stones. First, and I suspect you already know this but I'll include it here for completeness, quartz has a hardness of mohs 7, garnet is around 7.5, and SiC around 9.1-9.3. So garnet is harder than quartz and will work as a grinding agent, but SiC is much harder so will work better.

More importantly, SiC is hard but brittle, so it tends to gradually break down into finer sizes by fracturing into smaller but still very sharp pieces that cut aggressively. Garnet on the other hand, doesn't fracture, instead it wears away gradually, becoming more rounded and smaller... and more like ball-bearings than a cutting agent. To demonstrate this (ignore the different scales please!), here is 60/90 SiC, magnified with my macro lense -- nice and sharp, and you'll have to trust me that it continues nice and sharp as it fractures into smaller pieces ;D


And here is some garnet I tumbled -- it's much larger than your garnet grit, but gets rounded in the same fashion. After a short time in the barrel (maybe right from the start), the garnet just doesn't cut rock very well because it's round, not jagged and sharp.
-Don

Don, I never get tired of seeing that picture of your garnets. Such pretty, pretty lovelies.

"Garnet Sand: An excellent aid to grinding and polishing stones or glass in a vibrating or rolling tumbler. It's small size helps aid in grinding and polishing those tough to get areas. Use about 1/4 cup of garnet per quart of rocks. Use mixture from rough grind all the way to polish. Garnet sand 8/12 mesh." Page 37, Covington catalog. Looks like you would use it with S/C grit.