Choosing Rocks According to The Mohs Scale
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Every rock on the planet has its own place on the Mohs scale. This scale is what scientists and rockhounds use to determine the hardness of the rock, with talc being one and diamond being ten.
When choosing a few special rocks for the tumbler, keep this scale in mind. Rocks that register at 5, 6 or 7 on the scale are the best. Softer rocks are not usually worth the time and effort (note: Petoskey stone post coming soon) and harder rocks like diamond, emerald and sapphire are usually too precious so they are faceted instead.
Rocks similar to glass perform best in a tumbler. Agate, jasper, chert, petrified wood and other forms of non-chrystalline quartz will produce the best results. Obsidian, also known as volcanic glass, can do well. There are two ways to determine if a rock is good for the tumbler: the scratch test and fracture.
To scratch test a rock: With the blade of an ordinary pocket knife, scratch the rock. If the knife leaves a scratch mark, it is too soft for the tumbler. If the knife will not leave a mark, feel free to drop that rock into the tumbler. To further test the rock, see if it will scratch glass. If it will, it is a seven or higher on the scale.
Conchoidal fracture: Glasslike rocks do not simply break in half. Instead, they flake and leave a half-circle impression known as a conchoidal fracture. Sometimes when a windshield chips, this type of fracture is left (ironically caused by a rock in the road). If the rock already has this type of fracture on it, no other tests are needed. The picture above shows obsidian with a conchoidal fracture.
Next post: Grits
Categories: Tumbling
Tags: conchoidal fracture, hardness test, mohs scale, rock polishing, scratch test, tumbler
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Bringing Home Pretty Beach Stones
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As these May days get warmer people will head toward the beaches once again. Inevitably, bathing suit pockets and sand castle buckets will carry home pretty stones to remember the trip by. Unfortunately, too many times these colorful rocks sit on the dresser for a few weeks until dusting time, then they end up in the driveway or rock garden.
This year before leaving the beach with a bucketful of stones, take the time to sort them out and choose only the best ten or fifteen. Then cull that pile down to just the very best two or three. Once you have the best of the best stones in front of you, make that extra effort to turn them into something really special this year.
It is important to determine the hardness of the stone. Check out a Mohs scale to help with that. Michigan stones range from very soft alabaster to very hard agate. Next, decide if you want to wear the stone as a piece of jewelry or would like to display it as a specimen, paperweight or other decoration.
Tune in for the next post about tumbling, polishing and faceting.
Categories: Beachcombing
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