Gemology Fast Track / Unit 2 of 24 · Foundations · 60 min

Crystals and rough

Goal: Recognise the 7 crystal systems and typical rough shapes of major gems.

Cubic3 equal axes at 90°. Singly refractive. Diamond, spinel, garnet, fluorite.
Tetragonal2 equal + 1 different axis. Uniaxial. Zircon, scapolite, vesuvianite.
Hexagonal6-fold symmetry. Uniaxial. Beryl, apatite, benitoite.
Trigonal3-fold symmetry. Uniaxial. Corundum, quartz, tourmaline.
Orthorhombic3 unequal axes at 90°. Biaxial. Topaz, peridot, chrysoberyl, tanzanite, iolite.
MonoclinicOne inclined axis. Biaxial. Spodumene, orthoclase, jadeite, diopside.
TriclinicAll axes inclined. Biaxial. Labradorite, turquoise, kyanite, amazonite.
Amorphous / aggregateNo crystal order (opal, glass, amber) or many tiny crystals (jade, chalcedony).

Rough you will meet most

Corundum

Barrel or bipyramid, hexagonal outline; flat basal face with triangular growth marks. Often water-worn pebbles.

Beryl

Six-sided prism, flat ends. Emerald often in schist or calcite.

Spinel

Octahedron (two pyramids), often twinned flat (spinel twin).

Garnet

Dodecahedron (12 rhomb faces) or trapezohedron; rounded balls.

Tourmaline

Long prism, deeply striated along length, rounded triangular cross-section.

Quartz

Six-sided prism with pointed termination; horizontal striations on prism faces.

Topaz

Prism with lozenge cross-section; flat basal cleavage surface often present.

Diamond

Octahedron, dodecahedron, macle (flat triangle); greasy-adamantine; trigons on faces.

Chrysoberyl

Tabular, often 'sixling' (pseudo-hexagonal) twin with chevron striations.

Reading a water-worn pebble

Wet itWater or oil on the skin shows inner colour and clarity.
Window itA small polished flat lets you see inside and take an RI.
Weigh + measureSG on rough is easy and powerful: no setting, no glue.
Find the axisStriations, prism faces and pleochroism show the c-axis for orientation.

Rough gives you shape, cleavage, skin texture and SG, often enough to name it before cutting.

Practical
Search images for rough of each gem in the cards (corundum barrel, beryl prism, spinel octahedron, garnet dodecahedron, tourmaline prism). Then examine your own rough with the loupe: note faces, striations, and any flat cleavage planes.
Unit quiz

0/7 answered

1. Spinel rough typically forms…
2. Which system gives singly refractive (isotropic) gems?
3. Deep lengthwise striations and a rounded triangular cross-section suggest…
4. Corundum belongs to which system?
5. Jade (jadeite and nephrite) is…
6. Why wet rough before evaluating it?
7. Orthorhombic gems are…
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