Duncan Miller

Many faceters recommend that beginners start with aquamarine. It usually presents no problems in faceting or polishing, is relatively easy to obtain, and in lighter colour it is not overwhelmingly expensive. Aquamarine is the blue or blue-green gem variety of the mineral beryl, an aluminium beryllium silicate. It occurs in elongated hexagonal barrel-shaped crystals. It is dichroic, with the most intense colour when viewed along the length, the so-called c-axis. This is a pity, because the best recovery usually means placing the table facet parallel to the c-axis.

Selecting rough involves checking for inclusions. The crystals often have fine tubes running parallel to the c-axis. These can produce a sleepy but nevertheless pretty stone, perfectly suitable for a beginner’s practice. Avoid rough with irregular internal cracks. These will interfere with the light reflections and may cause the stone to break while you are faceting it. Light blue-green glass and blue synthetic quartz are common aquamarine simulants. The glass usually contains swirls of round bubbles; and is isotropic, staying dark on all rotations between crossed polarising filters. The blue synthetic quartz is more difficult to distinguish so it is best for beginners to stick to aquamarine rough in the characteristic hexagonal crystals, or bought from a reputable dealer.

Aquamarine is not particularly heat sensitive. After trimming off unwanted material I dop with wax, cut larger facets with 600 mesh followed by recutting them and cutting finer facets with 1200 mesh, and then polishing with aluminium oxide (Linde A) on a Batt lap. I used to use a tin/lead lap, which works just as well. Aquamarine traditionally is cut in elongated step cuts, like emerald cuts, because that is what fits well-shaped crystal rough. But trillions, round brilliants and fancy cuts with the table perpendicular to the c-axis can produce beautiful stones with stronger colour. Many greenish-blue aquamarines are heat-treated to remove the green component and produce more fashionably blue stones.

Other gem varieties of beryl include pink morganite, yellow heliodor, colourless goshenite, and green emerald. Morganite can be available in large pieces. Small stones can be nearly colourless and approach goshenite. None of the varieties present much trouble in faceting and polishing, although some morganite contains numerous microscopic inclusions that can make polishing without fine scratches difficult. Emerald is a different story. Apart from being very expensive, clear emerald rough usually is small. Larger pieces often are heavily included and don’t produce very attractive stones. There is a lot of fake emerald rough doing the rounds in South Africa. You need to be able to recognise these fakes to avoid being taken for a ride. A convincing fake consists of green glass that has been melted and poured into moulds lined with biotite mica. This mimics a common occurrence of emerald in biotite schists. Some of these have only five side faces – a dead give-away because emeralds form hexagonal crystals. Another common fake consists of dyed, quench-cracked quartz, but this is less attractive because of the crazing.

I preform emeralds on 600 mesh and facet them with 1200 mesh laps. They polish easily so a prepolish isn’t necessary. I used to polish them on tin/lead with Linde A, but both the polishing medium and bits of metal stuck in cracks and voids. This had to be cleaned out with dilute nitric acid. Now I polish on one of Gearloose Lapidary’s water-only polishing laps, either Greenway or Skyway. I find the Greenway, already impregnated with chromium oxide, works more rapidly, but it can round small facets a bit.

There is ongoing discussion among gemmologists if all green beryl gems should be called emerald. Typical emeralds are coloured by small amounts of chromium, often with some vanadium. Vanadium coloured beryls tend to be called emeralds these days, while those coloured mainly by iron tend to be called green beryl. There is no strict dividing line, and most of us are not experienced spectroscopists! So if in doubt, just call your greenish beryl gem ‘green beryl’ and you won’t be wrong.

Various rough beryl crystals (clockwise from top left) – goshenite, morganite, green beryl, two emerald crystals, aquamarine, heliodor (probably irradiated)