montana agate inclusions

If you've made it through the pages on "internal structure of Montana agate" and "Montana agate inclusions", you probably have an appreciation of the fact that Montana agate is NOT a homogeneous stone, and the beautiful internal features will tend to be oriented according to internal structures. Certain cut orientations may yield gorgeous slabs..... cutting the same agate at a different orientation may give you a slab with a few disappointing specks and poor patterning. More so than with most other stones, it really pays to think through how you're going to cut a Montana agate so that you can take maximum advantage of the inclusions.

Inspect the Stone

First, inspect the agate by wetting it and holding it under a bright light. Spend some time and thought to assess internal features carefully, and think about the best orientation to cut.

Does the rock have full-bodied dendrites ? Full-bodied dendrites tend to have a long axis..... cut parallel to the long axis.
Ribbon banding? Cut perpendicular to the banding.
Is the rock checked ? Try to cut parallel to checks so that you get more unchecked slab.
Does the rock have thin line dendrites? Try to cut parallel to the best, flattest, botryoidal surfaces. Consider cutting the slab a little thicker than usual, anticipating that you'll be able to shape the surface of the cabochon to take advantage of 3-D features in the agate.
Does the rock have flat, planar features ? Cut parallel to planes, try to “nick” the planes that have some dark features.
If the rock has a coarse crystalline center, you may get full-bodied dendrite “trees” at the intersections of botryoidal surfaces.

Cutting (top of page)

Some people start the first cut near the center of the stone to see what the interior looks like. I don't like to do this, since it's easy to buzz right through spectacular features. If you can't get an idea of what's inside the rock by wetting and using bright light, try grinding away some of the exterior of the stone so that you can see better. If you can see fingernail-sized, dark or black crescents on the surface, there's a good chance that the rock may have good internal features.

Al Siegel of Terry, Montana suggests that, for small or irregularly-shaped pieces, you can glue the stones onto wooden sticks to secure them in a rock saw carriage. I use 2” x 2” x 6” pine. Make up a thick mixture of Elmer’s glue & sawdust to attach the stone to the stick. Touch up the glue as needed. Let the glue harden for at least 3 days after the last application of glue. This works great so long as you aren't using a water-based lubricant for your saw. You can release the stone from the stick by soaking it in water overnight.

A rockhound in North Dakota told me to try using Mennen Pre-Shave to reduce oil mist. I couldn't stand the smell, so I experimented with rubbing alcohol.... this trick really works. I squirt in a few teaspoons of rubbing alcohol every four or five cuts. However, you have to let the alcohol evaporate from the reservoir before cutting rocks on sticks or else the Elmer’s glue may release and the rocks can fly off. I installed an oil circulating pump on my big saw, and I found that the alcohol trick doesn't work unless your saw uses the conventional oiling method, where the bottom of the saw blade dips into an oil reservoir at the bottom of the saw enclosure.

If, after the first cut, it looks like a different orientation may show internal features better, take the time and re-set the stone on a stick at a better orientation. For planar features, take your time….. if first cut is a bust, advance the rock only to near the next plane w/ dark features, since the planes with dark features may have layered dendrites. Use a very slow advance rate on your rock saw carriage.

Slabs (top of page)

Look for checks (cracks) in slabs. You may want to trace checks w/ a Sharpie pen so that you can better visualize the good areas in the slab. Don’t try to include unhealed checks in cabs.

If I'm working with a checked slab, I whack the slab on my bench surface fairly forcefully ….. the slab may break along weak cracks (this is especially useful for heavily checked rock, such as oolitic chert from central Pennsylvania).

For small slabs that don’t immediately suggest good cab features ….. tumble them and re-inspect them.... this may make features easier to see, and the slab may fall apart along weak cracks.

While it's tempting to go for the HUGE cabochon with spectacular features, also try to visualize what a small cabochon may look like when a small feature is cut away from the rest of the slab.

For free-form cabs, use a soft lead pencil to outline cabs. Once you’re satisfied with the result, re-trace w/ fine-point Sharpie pen. Trim the form on a trim-saw.

In the old days, people used wax, shellac, and other adhesives to hold a cab onto a dop (metal or wood stick - I prefer wood). I've found that high-temperature glue-stick (WalMart's finest) works beautifully, and is MUCH easier to work with than wax. However, the stone has to absolutely dry, or else the glue will not hold. Agate, being porous, will hold quite a bit of water after you immerse it in water while trimming or washing. I made up a small wire colander to hold stones over a 100 watt light bulb....... this drives out the water after an hour or so, and can also be used to release stones from the dop stick.

I was in our local Lowe's hardware store when they had a clearance on tile saws. I bought one, and figured out that, with a little modification, you can rough-shape cabochons nicely using the outside edge of an inexpensive tile saw..... this saves wear on my (expensive) #80 diamond grinding wheel. Just be sure to use enough water to control dust.

Hand Specimens (top of page)

One of my strongest memories from early in my love affair with Montana agate was visiting Oscar Mastvelten's shop in Glendive and seeing his hand specimens. If he cut a particularly nice agate, he might cut half or two-thirds of the agate into slabs, and he would shape a shallow crown on the sawed surface of the remainder...... this is a nice way to showcase your best stones, and also leaves enough crust on the stone so that you don't forget what the original gravel looked like.