Summary
Many faceters have told me they have trouble cutting corundum — ruby and sapphire. So, in this article, I’m going to show how I do it by cutting a “Simple Heart,” one of my new heart-shaped gem designs. I’ll use a piece of synthetic (lab-created) ruby, since you can buy this material very easily almost anywhere. Plus, I thought a heart for Valentine’s Day should be red.
Reading time: 10 min
Simple Heart - heart ruby design

You can find cutting information for Jeff Graham’s “Simple Heart” gem design here.

Preparation

I sliced this slightly rectangular piece of synthetic ruby off a corundum boule. Notice that I used a rough lap (100 grit) to cut a flat onto the split side of the boule. This makes dopping easier. Since the boules are almost polished where they’ve been split, doing this was necessary to get a good wax joint. The wax works better on a rough surface. I cleaned the surface well with alcohol before applying my lacquer solution (before dopping).

Synthetic ruby - heart ruby

Synthetic ruby

Next, I dopped the ruby. If your machine is keyed, carefully align the points of the stone to the shape of the heart design so you won’t waste rough.

Dopped synthetic ruby - heart ruby

Dopped synthetic ruby

I used a 100-extra grit lap to rough in this stone. (This lap has more diamonds than a regular 100-grit lap). This was a fairly large piece of synthetic ruby, and I wanted to get it done.  If this had been a smaller, natural corundum, I probably would have used a well-worn 260-grit lap for roughing it in. (That’s still a pretty coarse lap, especially for a smaller stone).

Please note that even though I used what most would call a very coarse lap, I still cut to a good centerpoint. Get into this habit. Always use a lap coarse enough to get the job done fairly fast, but also make sure you’re cutting accurately. Meet your centerpoints and other meets and don’t over-cut the facets. Over-cutting will cost you time and trouble later. Use a light touch so you don’t over-cut your facets. This will make your laps last longer (and your cutting time much more enjoyable). Nothing is worse than spending an hour roughing in a stone, when you could do it in 5 minutes with a coarser lap.

Pavilion

Important: make sure and cut P1 and P2 to centerpoint over the center of the dop. Since this heart ruby (like almost all hearts) has 1-fold symmetry, you could cut to centerpoint without being centered on the dop. P1 and P2 are opposites for easier centering during the initial cutting.

The top of the heart is 12 o’clock in all of the pictures.

P1 and P2 facets - heart ruby

P1 are P2 facets are cut to center point.

Cut facets P3, P4, P5, and P6 to centerpoint.

P3 and P5 - heart ruby

Facets P3 (left) and P5 (right) are cut to centerpoint.

Girdle

When you cut the girdle facets to meet the pavilion tiers, make sure you cut them as evenly as you can. A mistake here — especially over-cutting — can cost you a lot of time and wasted rough.

P7(G) facet - heart ruby

Cut P7(G) to meet P1 point.

Tip: I use the coarse lap to get close to where I think the final girdle line needs to be. However, I leave the girdle just a little shallow and cut it to the proper depth on a finer lap.

P8(G) to P12(G) - heart ruby

Cut P8(G) to P12(G) to meet their respective pavilion tiers, forming the heart outline.

Pavilion

Cut P1 and P3 to centerpoint with a 600-grit lap. On a smaller, natural corundum, I would usually go from a worn 260-grit lap to a worn 1,200-grit steel lap. Since this is a large synthetic ruby, however, I’m going to a 600-grit steel lap from the 100-extra grit lap. This will make the cutting go faster.

P1 and P3 cut to centerpoint - heart ruby

P1 and P3 cut to centerpoint with a 600-grit lap.

I cut to meet all the pavilion and girdle facets pictured below with a 600-grit lap.

Facets cut to meet with 600-grit lap - heart ruby

Facets cut to meet with 600-grit lap.

Next, I used a 1,200-grit steel lap to fine cut all the facets before polishing. Note that I cut in P13 with the 1,200 fine lap.

All facets fine cut with a 1,200-grit steel lap - heart ruby

All facets fine cut with a 1,200-grit steel lap.

Corundum Pitting

Here is where most faceters start to have problems.

Corundum pieces will typically be slightly harder in different directions (on different facets). Some facets will also pit on a fine lap. This is normal. In the picture below, you can see that one of the girdle facets is definitely pitting.

Typical corundum pitting - heart ruby

Typical corundum pitting

Although less severe in synthetic corundum, these traits are still present. The pits typically show up on a fine lap. You usually won’t see them on a 600-grit lap or lower. Some faceters try to cut just to a 600-grit lap and polish from there. (Note that 600-grit NuBond will usually pit).

Personally, I don’t feel this is the right thing to do. The pitting is part of the crystal structure. The pits are going to be there, whether you cut to a 600 grit or a 1,200 grit pre-polish. (That has been my experience). I think you’ll still need to polish through the pits either way.

However, you’ll get a good polish from a 1,200 pre-polish much easier and faster. If you try to polish from a 600-grit pre-polish, you’ll be polishing for a long time. Of course, if you want to do it from a 600, feel free. I usually leave the facets that are pitting just a little bit short of meetpoint. That way, when I polish out the pits, I don’t over-cut that facet.

Polishing

I polish all my corundum pieces using first an 8,000 and then a 50,000 diamond polish on zinc laps. For me, I’ve found this the best combination. 

I should point out that I cut very little synthetic corundum. Faceters tend to cut this material into stones larger than would be possible with most available natural ruby or sapphire. If I was going to cut a lot of large synthetic corundum pieces, I might consider adding a 3,000 diamond/zinc lap.

In the pic below, you can see that the 8,000 pre-polish with a zinc lap got the pits out of the middle (shiny) girdle facet. It did the job very quickly, too.

Girdle facet after 8,000 pre-polish - heart ruby

Girdle facet after 8,000 pre-polish

I usually run my zinc laps pretty fast (¾ to full speed) when I polish ruby or sapphire (and chrysoberyl, too). If the lap slows down, I give it a good shot of diamonds with Crystalite pump bottles. I’m not shy about using more diamonds with corundum on zinc laps. These laps have never given me any scratching problems.

Pavilion and girdle polished to 50,000 - heart ruby

Pavilion and girdle polished to 50,000.

You must clean the pavilion and girdle very well to get rid of any oil/lube left over from polishing. I use alcohol for this. Otherwise, the wax won’t stick when you transfer.

Transferred stone - heart ruby

Transferred stone

Crown

Cut C1 through C6 to meet girdle with a 600-grit lap. Generally, I’d use a 600-grit steel lap at this stage no mater what type of material I’m cutting. A 600 is fast but fine enough to get the cheat set (if needed) and the facets lined up with the girdle. I line up and cut the crown, then I cut the girdle to the proper thickness, maybe just a hair thicker than I think I’m going to want it when the stone is finished.

Cut C1 through C6 to meet girdle - heart ruby

Cut C1 through C6 to meet girdle.

Crown Break Facets

Next, I cut all the crown break facets with my 1,200-grit steel lap.

Crown break facets - heart ruby

Crown break facets

I do this for two reasons. First, the cheat, especially on a larger stone, often needs just a nudge to line up everything when you cut the breaks with a fine lap. Second, I tend to cut all the rest of the facets to meetpoint with a 1,200 lap because it’s finer, even though it cuts a little slower. At this stage, it’s better to go slower than cut through a meet and have to re-cut the entire crown again.

Cut C7, C8, and C9 to meet girdle using a 1,200-grit lap - heart ruby

Cut C7, C8, and C9 to meet girdle using a 1,200-grit lap.

Cut in C10 (left) to meet girdle and C11 (right) to meet C6 using a 1,200-grit lap - heart ruby

Cut in C10 (left) to meet girdle and C11 (right) to meet C6 using a 1,200-grit lap.

At this point, look carefully at your stone and see if all your meetpoints are correct. Also, look for any facets that seem to be pitting or don’t look like the others. Cut them a little short of the meetpoint(s), if you find any.

Below, you can see what the crown should look like with all the facets cut in. If your crown looks different, then you may have a problem. Notice that the facets all look the same on this stone. No pitting (yet).

All crown facets are cut in - heart ruby

All crown facets are cut in.

Cutting the Table

At this point on most other types of stones, I would go ahead and start polishing the crown. I would polish from the crown breaks in towards the table, leaving it for last.

However, this is corundum.

With this material’s typical pitting and hardness variations in different directions, I always cut the table at this point. Why? Well, what do you think are the odds that pitting will appear on the table? If you assume it won’t, I’d say about 100%. On this particular stone, I had a good hint. I hadn’t encountered any pitting anywhere else on the crown.

So, guess what?

Table cut with pitting - heart ruby

Table cut in with a 1,200-grit steel lap. Notice the pitting.

Well, the odds were 100% that I would have pitting on the table when I cut it in.

This stone finished out to be 11.8 mm x 11.1 mm x 7.4 mm deep.

Even though this stone is pretty large — and the table is correspondingly large — the pits aren’t really a problem. The key is to know ahead of time that it’s pitting and cut the meets a little short. If I had polished the crown and then cut in the table, it would have been much more difficult to get all of the meetpoints on the table to meet correctly.

Polishing the Table

This is what the table looked like after I polished it with my 8,000 zinc lap. As you can see, the pits posed no problem. The table actually only took a couple of minutes to polish.

Table polished, pit free - heart ruby

Table polished with 8,000 diamond and zinc, pit free.

At this point, all I had left to do was polish the table with 50,000 and zinc (while I still had the same setting).

Next, I started on the crown breaks and polished them all in with my 8,000 zinc. Once done, I went back and polished all of them with my 50,000 zinc diamond lap. The 50,000 really makes the facets “pop.” Some faceters only polish to 14,000 diamond when they cut corundum. With commercially cut ruby and sapphire stones, this is actually pretty standard. However, I think the 50,000 polish works much better and recommend that faceters use it.

Why Start with Synthetic Corundum?

Once you’ve cut a couple of lab-created rubies or sapphires and learned how to deal with their idiosyncrasies, you’ll find them not too bad to do. I’ve found synthetic corundum easier to cut than most natural corundum. The synthetic material seems more “docile,” generally speaking. I suspect this is because it’s not subjected to the extremes of nature while forming. Also, synthetic corundum is pretty much a breeze to orient. Synthetic corundum stones in some colors do need orientation. For example, blue and orange tend to have their strongest color zones out near the rind of the boules. You’ll need to put the pavilion down into the color zone on these boules.

Other than that, synthetic corundum is inexpensive and predictable to cut. I recommend faceters cut some synthetic ruby or sapphire first before trying more expensive, natural rough.

Assuming you can even find decent rough, natural corundum will be much more expensive and much more difficult to cut, too. Orienting natural ruby or sapphire often takes an expert. Cutting these stones is usually unpredictable. Nevertheless, I recommend faceters cut some natural material after trying the synthetic. Natural rubies and sapphires make beautiful and unusual — not to mention valuable — faceted gemstones.