Gem Materials - and Some Properties

Natural Gemstones

Name
Hardness
Sp. Grav.
R.I.
Dispersion
Comments

Calcite

3
2.71
1.48-1.65
0.017

Too soft for most jewelry, 3 good cleavage planes, heat sensitive, chemically sensitive, start with this and you may never facet again.

Sphalerite

3.5-4
4.08-4.1
2.36-2.37
0.156

Not as bad as calcite, too soft for most jewelry, 6 directions of cleavage, great dispersion, hard to get facet rough.

Fluorite

4-4.5
3.18
1.434
0.007

Maybe easier than calcite (but not much), 4 perfect cleavages, even hard to dop and un-dop, too soft for most jewelry.

Glass

5
3.15-4.2
1.48-1.5
0.010

Little value, but easy to cut and polish, subject to some chipping. Too soft for rings.

Apatite

5
3.17-3.23
1.63-1.65
0.016

Too soft for rings. Heat sensitivity, a little hard to polish. Fracture free rough hard to find.

Feldspar
orthoclase
sunstone
labradorite
moonstone

6-6.5
2.56-2.6
1.52-1.53
0.012

Marginal hardness for rings. Cuts nicely, perfect cleavage, but not usually a problem. variety: Sunstone needs orientation to get good color. (color zoning)

Kunzite

6-7
3.16-3.2
1.65-1.68
0.017

Perfect cleavage can be a problem, fracture free rough is hard to get, polishing can be difficult. Cleavage and hardness make them marginal for rings.

Hiddenite

6-7
3.16-3.2
1.65-1.68
0.017

Peridot

6.5-7
3.27-3.36
1.65-1.69
0.020

Good durability, fairly easy to cut, but polishing can be difficult depending on the particular stone.

Zoisite
tanzanite

6.5-7
3.28-3.35
1.69-1.7
0.030

Tanzanite (bluish zoisite), good durability, easy to preform and polish. Fracture free rough, and any larger size rough is hard to get and expensive.

Zircon

6.5-7
3.9-4.7
1.77-1.98
0.039

A good material with no bad characteristics, some rough is too dark and will not produce good gems. High dispersion means a very fiery stone if not too dark.

Quartz
amethyst
citrine
smokey
clear

7
2.65
1.54-1.55
0.013

Well behaved, easy to work with and inexpensive. Can be somewhat trouble some polishing on ceramic laps. Use Linde A disks for easy polish. Good starter material - real or man-made.

Iolite
cordierite
(water sapphire)

7-7.5
2.6-2.66
1.53-1.55
0.017

Only difficulty is orienting the rough as it is trichroic. Cuts and polishes well. Larger rough hard to find, often fractured.

Garnet
almandine
rhodolite
tsavorite
grossular
demantoid
etc. etc.

7-7.5
3.4-4.2
1.73-1.89
0.024-0.027

Garnets are typically easy to cut and polish. There is such a wide variety of types that a summary here is impossible. My only warning would be to look closely at very dark stones, they may not look good when finished (almost black).

Tourmaline
chrome
indicolite
rubellite

7-7.5
3.02-3.263
1.61-1.65
0.017

Good material, cuts well, but one good cleavage plane. Slightly heat sensitive, and some rough will spit along flaws. Beautiful color spectrum.

Andalusite

7.5
3.12-3.18
1.64-1.65
0.016

A little brittle, so chipping can occur. Must be oriented to give pleasing color (trichroic material). One good cleavage plane but usually not a problem.

Beryl
aquamarine
emerald
morganite

7.5-8
2.65-2.75
1.57-1.60
0.014

Good material at all stages, nothing special to worry about except with Emerald (flaws and internal weakness). A little expensive but large, clean rough is available.

Spinel

8
3.58-3.61
1.71-1.74
0.026

Another good material with little difficulty. Rough can be pricing, but excellent man-made is available.

Topaz
blue
imperial
white

8
3.53-3.56
1.61-1.638
0.014

Some care needed in orienting the cleavage plane at about 10+ degrees off of the table. If not oriented properly it may break or become very difficult to polish. Produces clear gems, large rough in some types is easy to get.

Chrysoberyl
alexandrite

8.5
3.7-3.72
1.74-1.75
0.015

No problems easy to polish but can be slow. Real problem is getting any rough!

Corundum
ruby
sapphire

9
3.99-4.08
1.77-1.78
0.018

No problems easy to polish but can be slow except on diamond laps. Real material tends to be pricey, but lots of man-made available in all colors. Very durable.

Diamond

10
3.47-3.55
2.42
0.044

Hard to cut and polish (I wonder why?), wears out laps quickly, rough is VERY expensive, and not readily available.

Synthetic Gemstones

Strontium Titanate

6-6.5
5.13
1.83
0.190

A brittle material that produces gems with INTENSE color fire. Chips easily especially at the polish stage. Not easy to get any rough, off market for a long time.

Quartz

7
2.65
1.54-1.55
0.013

Well behaved, easy to work with and inexpensive. Can be somewhat trouble some polishing on ceramic laps. Use Linde A disks for easy polish. Good starter material - real or man-made.

Spinel

8
3.58-3.61
1.71-1.74
0.026

Another good material with little difficulty except in polishing. Rough can be pricing, but excellent man-made is available.

YAG

8
4.6
2.4-2.42
0.028

Easy to cut, polish and work with. Hard to find anymore as it has been replaced with CZ.

Cubic Zirconia

8.5
5.7
2.15-2.18
0.060

Not expensive but high density makes for heavy stones, easy to cut and preform, but polish can be tricky. Does not do well on ceramic laps, does polish well with diamond on a "fast Lap" from Raytec.

Corundum

9
3.99-4.08
1.77-1.78
0.018

No problems easy to polish but can be slow except on diamond laps. Real material tends to be pricey, but lots of man-made available in all colors. Very durable.

Silicon Carbide

9.25
3.21
2.65
0.104

Gem name (Moissanite), apparently cuts well on diamond, produces stones with excellent fire, but rough is very hard to get.

Some general comments on the above materials.

Starting faceters usually select synthetic materials or quartz as good practice material. Synthetic corundum or spinel is easy to get in a number of colors and most will cost only a few cents per carat ($0.12-$0.20 per carat).It polishes well on ceramic laps but may be difficult on others. Cubic Zirconia is about the same ($0.10-$0.25 per carat), but can be a little tricky in the final polishing stages. Because of the polish I do not recommend CZ or spinel for beginners.

Quartz, both synthetic and natural can be obtained without much difficulty. The synthetic runs only a few cents per carat in the rough ($0.05-$0.12 per carat), but the real materials vary according to their quality and color. You can get rock crystal (clear quartz) for about the same price as synthetic, citrine (yellow), and amethyst (purple) quartz are more expensive as natural rough. Citrine and amethyst can be obtained in good (but not best colors) for between about $0.25 and $0.60 a carat. Top quality citrine and amethyst might fetch $1 - $3 a carat, but I would not pay much more than that for any quartz rough.

Synthetic quartz is available in shades of blue, green, and even ametrine (citrine-amethyst mix). Glass is also a good beginners material, look for older quart size glass 7up bottles, their bases make nice stones. Another good source for glass rough is marbles. Look at JoAnn Fabrics, or Michael's Craft supply for glass marbles or decorative glass cubes.

Light colored beryl is also inexpensive and works up easily. You can find it in the rough, for $1 to $3 per carat in the light yellow, or clear varieties.

YAG can still be found, but costs a little more ($0.30 per carat).

In the charts above, those materials with darkened comment boxes (green) are recommend as good for most beginners or early faceters, those in red should need to be carefully considered before attempting. Moissanite is simply not readily available in the rough, and diamond takes special equipment even if you can get it. It will tear through hobby laps.

Now as to the numbers, I consider any stone with a hardness of 7 or greater to be good for a finger ring. Many people do daily work wearing a ring, and if the stone is less than 7 there is a fairly good probability that it will be scratched on something. The one exception might be a stone like topaz, which is hard enough, but may be brittle in a ring, and somewhat more shock sensitive.

To cut a fiery stone you need to select materials which are both light in color, and have a high dispersion. Really dark stones, like deep red zircons have high dispersion, but lose it in their color depth. Fiery stones look best when they are light in color, and more transparent.

Stones being cut to show off their color, should be cut with minimal facets and large facets. You are trying to show their color, not their reflectance, sparkle, or fire. The dark red variety of garnets are good at this, as well as emerald, and some of the dark tourmalines. Dark stones show their best effect when we see the richness of their color, not their ability to sparkle around the edges.

Some cuts simply need more stone. Do not try facet diagrams with a hundred plus facets on little stones, the effect will be lost. Small stones (6 mm and less in diameter) need simple cuts up to about a round brilliant cut (57 facets), and more complex cuts require a bigger gem. If you want to show off a Portuguese cut or a whirlpool cut then be prepared to cut a stone at least 10-15 mm or even larger.

Remember, you can produce some really nice stones with very few facets. A good design, and well polished surfaces can make a gemstone look really good. Especially when put up against the run-of-the-mill stone cuts seen in many jewelry and department stores. Most of the worlds bulk cut stones are done my native faceters with primitive equipment. They are trained to cut the largest size stone from the rough, not to give the most brilliance or quality. These "commercial cuts" often suffer from poor design and never look good against top quality simple cuts.

Later we'll look at some commercial cuts (native cuts) and examine some of the more prevalent problems.