Selecting Rough
June 25, 2016By Duncan Miller
If you are
going to facet, you need to learn something about mineralogy because you need
to know what stones you should obtain, how their characteristics affect their
behaviour while you are cutting and polishing them, and how they affect the
optical properties of your finished gemstone. The easiest material for beginners to cut and polish is common red garnet. It
presents no problem with cleavage or orientation for colour, and generally
behaves itself well during ‘cutting’ and polishing. The only drawback is that
most common red garnet is very dark, and so unless you make it small your final
stone also will be dark. A good alternative is aquamarine. Even lightly
included stones will give you a pretty gem, and it has no problem with
cleavage. High quality, expensive aquamarine with a deep blue colour needs to
be orientated properly for colour, but that is not the sort of material you
should be cutting at first. Many people start faceting with quartz. It is cheap
and readily available, so it is not a bad choice. Some people find quartz
difficult to polish, but with an appropriate combination of lap and oxide
polish it should not be problematic.
Colourless topaz is also fairly readily
available and cheap, but with topaz you need to orientate your stone to avoid having
any major facet, and in particular, the table facet, parallel to the cleavage plane. In well-shaped crystals
the cleavage plane direction usually is easy to determine, and your stone must
be orientated with the table facet at least five degrees off the cleavage
plane. With water-worn pebbles of topaz it can be difficult to identify the
cleavage plane and you will have to ask someone with experience to help you.
You also have to be able to distinguish water-worn topaz from water-worn
quartz, either by relative hardness or density measurement. Topaz is harder in
most directions than quartz, and it is more dense. But you need some experience
in determining these differences.
In selecting rough you have several things to look for, apart from mineral species. Rough needs to be a suitable shape. Long needle-like crystals are difficult to facet without fracturing them. Thin, flat pieces of rough do not have enough depth to produce a good stone. Chunky pieces of rough will give you better recovery than pieces with irregular protrusions.
You need to look for inclusions and flaws. Immersing the rough in water, baby oil, or a mineral oil like ‘liquid paraffin’ sold as a laxative, will allow you to see the interior to inspect it for flaws and inclusions. Do not hold the rough up to a strong light and peer directly at it to try to see inside it. Rather hold the stone under the edge of a shaded lamp, or shine a penlight torch through it from the side or beneath, but ensuring that the lighting is perpendicular to your line of sight. The extent of inclusions that make an acceptable stone is entirely up to you. Some inclusions, like rutile needles, can enhance a gem. Large numbers of fractures can weaken a stone and interfere with the passage of light. Small inclusions can be ‘hidden’ underneath facets near the girdle, where they will not be so obvious.
To assess the colour of coloured rough, again do not hold it up to a strong light a look through the stone. This will give you a completely false idea of the colour of any gem you cut from it. Place the rough on a piece of white paper or a pocket mirror and inspect it in strong, but indirect white light. Don’t direct the light through the ‘back’ of the stone, but look at the colour reflected back through the stone. If you can’t see a desirable colour with this ‘white paper test’, the stone is not worth cutting.
Uneven colouring needs to be orientated when you plan your stone. A strong band of colour in an otherwise colourless stone can be orientated parallel to the table to enhance the colour of the finished stone. A spot of deeper colour can be positioned in the middle of the stone or deeper towards the bottom point or keel, to reflect throughout the stone.
Many minerals show pleochroism, that is, they have different colours when viewed in different directions. Tourmaline is a good example, usually having a stronger or different colour when viewed down the length of an elongated crystal as opposed to across its width. Where possible given the shape of the rough, strongly pleochroic stones need to be orientated to produce the most desirable colour when viewed through the table facet.
This all may sound a bit much, but fortunately you don’t usually have to take all these issues into account for any one stone. Shape and clarity probably are your main concerns to start with, then orientating for colour and cleavage. Sometimes you have to compromise. All the time, you have to keep learning.
Ametrine rough – quartz with different sectors coloured yellow and purple, a real puzzle when it comes to orientating the rough, in this case try to capture both colours when viewing the stone through the table.
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