C.1 The eleven physical properties
Minerals are identified by physical properties because, as § A.1 explained, a fixed chemistry and lattice produce consistent, measurable characteristics. The eleven commonly used (a twelfth, magnetism, is often added) are:
(a) Crystal form. Minerals are built from groups of atoms, and the atomic arrangement governs the crystal's form (shape). Different chemistries give different atom groups and therefore different crystal forms — so the form is diagnostic. Diamond, in the cubic system, can appear as octahedra; quartz forms hexagonal prisms.
(b) Habit. The general appearance a mineral tends to take — blocky crystals, long slender ones, or aggregates. Glassy but cubic crystals, for instance, tell you the specimen is not quartz. Habit is the "body language" of the mineral.
(c) Hardness. One of the most important properties: resistance to scratching, measured by comparison against the Mohs Scale (§ C.2). Hardness is consistent for a given mineral and so is a primary diagnostic.
(d) Specific gravity (SG). The weight of a substance in air divided by the difference between its weight in air and its weight when immersed in water — i.e. density relative to water. SG is powerful for minerals because the heavy ones (gold SG 19.3) are dramatically denser than ordinary rock (SG ~2.6), so "weight-for-volume" is an immediate field signal.
(e) Colour. A highly variable characteristic that cannot be relied on alone — impurities change the colour of one mineral, and different minerals share colours. Colour is a starting hypothesis, never a conclusion (see Pitfall H.1).
(f) Streak. The colour of the mineral's powder, obtained by grinding/scratching a small amount on a porcelain streak plate. The streak may differ markedly from the colour of the mass and is more reliable than colour, because the powder removes surface and crystal-size effects.
(g) Lustre. The appearance of a fresh surface when reflecting light. Types include metallic (like polished metal, e.g. gold), sub-metallic (imperfect metallic, e.g. tantalite), and adamantine (brilliant, like diamond). Lustre quickly separates metallic minerals from non-metallic.
(h) Cleavage. The tendency of a crystallised mineral to break along definite directions, yielding smooth cleavage planes. A perfect cleavage gives smooth, lustrous surfaces easily — which is why even diamond, the hardest mineral, can be cleaved with a hammer and chisel along its cleavage plane (exploited by diamond cutters).
(i) Fracture. A break other than along a cleavage — the surface produced when the mineral does not split cleanly.
(j) Fusibility. The relative ease with which a mineral melts on heating. Stibnite fuses in a candle flame; garnet needs ~1,200 °C. Fusibility separates low-melting from refractory minerals.
(k) Fluorescence. Some minerals emit light under ultraviolet light (e.g. scheelite) — a UV lamp is a simple field discriminator.
(l) Magnetism (the twelfth). Certain metallic minerals are magnetic (e.g. magnetite, and to a lesser extent pyrrhotite). A magnet is a cheap, decisive field test for these.
The officer's basic field kit — a Mohs reference set or proxies (fingernail ~2.5, copper coin ~3, knife/glass ~5.5), a streak plate, a magnifying lens (loupe), a magnet, and access to acids for confirmatory chemistry, plus a UV lamp — lets these properties be tested at the border. For high-value or contested cases, the consignment is referred to MMCZ technical staff or an accredited laboratory (e.g. micro X-ray fluorescence, µXRF) for definitive analysis.
C.2 The Mohs Scale of Hardness
The Mohs Scale ranks ten reference minerals by increasing hardness; an unknown is placed by seeing what it scratches and what scratches it:
| Mohs No. |
Standard mineral |
Field proxy |
| 1 |
Talc |
scratched by fingernail |
| 2 |
Gypsum |
scratched by fingernail (~2.5) |
| 3 |
Calcite |
scratched by copper coin (~3) |
| 4 |
Fluorspar (fluorite) |
— |
| 5 |
Apatite |
scratched by knife (~5.5) |
| 6 |
Orthoclase feldspar |
scratches glass |
| 7 |
Quartz |
scratches glass readily |
| 8 |
Topaz |
— |
| 9 |
Corundum |
— |
| 10 |
Diamond |
scratches everything; scratched by nothing natural |
A fingernail scratches talc and gypsum but not calcite; a knife blade scratches apatite but not feldspar. The scale is the international standard, valued precisely because it needs no instrument — only comparison — which makes it ideal at the border.
C.3 Gold
What gold is. Gold is a metal, gold/yellow in colour, that "may look like brass". The decisive difference: brass dissolves in sulphuric or nitric acid; gold does not (gold resists individual acids and dissolves only in nitrohydrochloric acid / aqua regia). Local names include mukute, igolide, chuma, "stuff", "yellow".
The seven forms of gold. The officer must know each, because gold presents very differently depending on stage:
- Alluvial gold — fine particles deposited in riverbeds, washed down by water; average ~98% purity; recoverable without sophisticated machinery.
- Gold concentrate — pulp or mortar with very high gold concentration.
- Reef gold — gold embedded in rock, requiring equipment to extract.
- Gold nuggets — large natural pieces.
- Gold amalgam — an alloy of gold and mercury.
- Retort — the product of an amalgam after the mercury has been removed.
- Gold bullion — the product of smelting (refined bars).
Characteristics and tests. Gold is the most malleable and ductile of metals — beaten to sheets or drawn to wire without breaking. Melting point 1,063 °C; an excellent conductor of heat and electricity; yellow where most pure metals are grey/silver. Diagnostic triad: specific gravity 19.3 (weight feels disproportionate to volume), hardness ~2.5 (soft, fingernail-scratchable), and acid behaviour (resists nitric/sulphuric; dissolves in aqua regia — distinguishing it from brass, which dissolves, and from pyrite/"fool's gold", which is harder and brittle).
Customs significance. Gold is Chapter 71 of the HS (precious metals). Export is restricted to the RBZ/Fidelity Gold Refinery channel; gold royalty is tiered and price-linked (Twenty-Fourth Schedule; § B.3). Identification at RGM International Airport is the front line against the country's most intensive smuggling flow.
C.4 Diamonds
What a diamond is. From the Greek adamas, "indestructible". Chemically carbon (C) in the cubic crystal system; the hardest known mineral (Mohs 10). Rough diamonds are un-worked (un-cut) stones.
Recovery — and why it shows in the shape. Two recovery settings, which the officer must distinguish:
- Mine (pipe) deposits — kimberlite. Kimberlite is rock from a volcano that burst to the surface, cutting through and carrying chunks of overlying rock in which diamonds are found. These diamonds have not been eroded or weathered, so they are typically angular, showing classic crystal shapes with clearly defined faces and edges.
- Placer deposits. Diamonds moved from their source rock by erosion and other forces, coming to rest in river beds, beaches or sea floors. Transport scratches, bumps and rounds the edges, so placer diamonds tend to have worn, rounded edges like stream pebbles, and irregular/elongated shapes.
Types. (a) Natural — formed in nature, recovered by mining on land or in water; (b) Synthetic — laboratory-made, usually uniform in size and colour (a key tell).
Properties and tests. (a) Hardness 10 — scratches glass, quartz and corundum (the scratch test). (b) Cleavage — despite its hardness, it cleaves easily along its cleavage plane (cutters exploit this). (c) Specific gravity 3.52 — about a third denser than typical rock. (d) Colour — all shades but mostly white, yellow or brown. (e) Lustre — brilliantly adamantine. (f) Repels water but readily accepts wax and grease. (g) Thermal conductivity — an excellent heat conductor (about 4× copper); touched to the lips it feels ice-cold because it draws heat away. The three field-test pillars are hardness, specific gravity and thermal conductivity.
Rough diamond shapes. Octahedron (eight-faced double pyramid), dodecahedron (twelve-faced), cube (rare in gem quality), whole (rounded modifications of the above), and irregular (elongated, rounded — common in river-bed/alluvial placers).
Fake-diamond field tests. (i) Newspaper test — a real diamond placed on newsprint does not magnify the text (it refracts, it does not act as a lens); a fake (glass) magnifies. (ii) Light test — focused light passed through a real diamond produces a halo; through a fake, the light passes straight through and is visible on the far side. Both rely on the diamond's refractive behaviour.
Customs significance. Diamonds are Chapter 71; rough-diamond export requires the MMCZ certificate plus the Kimberley Process certificate. The officer verifies both and tests the stones.
C.5 Emeralds
The word "emerald" derives from the Greek smaragdus, "green"; emeralds are green by definition. An emerald is a green variety of the mineral beryl, coloured green by trace chromium (and sometimes vanadium). Specific gravity ~2.6–2.8.
Field tests for a real emerald (from the source): (a) Water test — a real emerald in a glass of water radiates green light; an imitation does not. (b) Eye test — a real emerald placed on the eye gives a cool feeling; an imitation warms quickly. (c) Soft-wood test — rubbed slowly on soft wood, a real emerald shines; a fake shows no change. (d) Light reflections on a small white cloth, magnifying-lens inspection (for inclusions), and a water-drop test supplement these, with SG 2.6–2.8 as a confirmatory measure. Synthetic emeralds are increasingly common and often require laboratory confirmation (Pitfall H.3). Emeralds fall under Chapter 71 (precious/semi-precious stones) and are MMCZ-controlled.
C.6 Lithium-bearing minerals
Strategic importance. Lithium is the battery-era strategic mineral; Zimbabwe holds significant LCT-pegmatite resources, and lithium is squarely within the mineral export levy net (§ B.3). The officer must recognise the six economically relevant lithium-bearing minerals:
| Mineral |
Distinctive features |
| Lepidolite |
Pink/red/purple mica (sheet-like cleavage); lithium-rich; commonly with rubidium/cesium; the most common lithium mineral |
| Petalite |
Colourless/white/grey/pink; tabular prismatic; vitreous lustre |
| Spodumene (Triphane) |
Pyroxene; colourless/yellowish/pinkish/lilac; large prismatic crystals |
| Amblygonite |
Milky-white to salmon-pink; sub-vitreous to resinous lustre |
| Eucryptite |
Lithium-aluminium silicate; less commonly economically mined |
| Zinnwaldite |
Lithium-iron mica; intermediate between muscovite and biotite |
Occurrence. About 44% of lithium minerals occur in LCT-pegmatites (LCT = Lithium-Cesium-Tantalum) and associated metasomatic rocks; others in non-LCT pegmatites, manganese deposits and large salt lakes. Associated elements in LCT-pegmatites include gallium (Ga), rubidium (Rb), niobium (Nb), tin (Sn), tantalum (Ta) and titanium (Ti) — and by analysing these (e.g. by µXRF), satisfactory lithium content can be predicted. Associated minerals include tantalite, pollucite, beryl, microlite and cassiterite — relevant because some are separately strategic/controlled. Lithium ores classify in HS Chapter 25 or 26 depending on processing; the species and Li₂O grade matter for both valuation and levy/royalty.
C.7 Coal and coke
Coal is a combustible raw mineral, typically black, low-value/high-volume, used for electricity and industrial heat. Coke is a by-product of coal — coal baked in ovens, heated in the absence of air (the coking process, >600 °C); it is grey, hard, porous, burns without smoke, has more energy than coal, and is used to smelt metals (copper, iron, steel, cobalt).
Identification and pricing. Run-of-mine product is crushed and screened by size, and priced by the percentage of chemical elements: moisture, calorific value, ash, fixed carbon, sulphur, phosphorus. Grades (sized in mm) include, for coal: Peas 7–40, Nuts 28–50, Large Cobbles 40–120, NPD Low Ash 0–50, Fines −5, Flints 3–10, Duff 4–10, Thermal 6–30, Slurry −5; and for coke: Coke Breeze −10, Coke Fines −5, Foundry Coke +80, Met Coke 40–80, Coke Nuts 20–60, Coke Peas 10–40. Because grade drives value, mis-declared grade produces under- or over-valuation (Pitfall H.5). Coal/coke classify in HS Chapter 27; coal carries 2% royalty (REV 5C) and the specified-minerals levy (§ B.3).