Iron Ore Types
Part of Metalworking
Identifying different iron ores in the field to find usable smelting feedstock.
Why This Matters
Iron is the foundation metal of civilization. It makes plows, axes, knives, nails, hinges, saws, and a thousand other tools that separate subsistence survival from productive community life. But iron does not appear in nature as shiny metal bars — it is locked inside minerals mixed with rock, clay, and sand. You need to recognize which rocks contain enough iron to be worth smelting and which are waste.
The good news is that iron is the fourth most abundant element in Earth’s crust. Usable iron ore exists on every continent, in every climate zone. The challenge is not finding iron — it is recognizing it. Iron ores come in many forms, colors, and concentrations. Some are obvious (heavy, rust-red rocks); others are subtle (black sand in a stream bed, yellowish-brown clay deposits).
Knowing your ore types also tells you what to expect during smelting. Different ores have different iron contents, different impurities, and different processing requirements. Hematite smelts differently from bog iron, which smelts differently from magnetite sand. Matching your smelting technique to your ore type is the difference between producing usable iron and producing a pile of slag.
Major Iron Ore Types
Hematite (Fe₂O₃)
Iron content: 50–70% Color: Steel grey to black (specular), red-brown to rust-red (earite) Streak: Cherry red to reddish-brown (always — this is the definitive test) Hardness: 5–6.5 (scratches glass) Density: Very heavy — noticeably heavier than surrounding rock
Hematite is the most important iron ore worldwide and the easiest to smelt. It reduces to iron at relatively low temperatures and produces good-quality blooms with manageable slag.
Field identification:
- Scratch the rock on an unglazed ceramic surface (a broken pottery shard works). If the streak is red-brown, it is almost certainly hematite regardless of the rock’s surface color.
- Heft — hematite is conspicuously heavy for its size.
- Specular hematite has a metallic, mirror-like sheen on fresh fracture surfaces.
Red Soil = Hematite Nearby
If the soil in an area is distinctly red or rust-colored, there is weathered hematite in the local geology. Follow the color intensity uphill to find the source deposit.
Magnetite (Fe₃O₄)
Iron content: 60–72% (highest of any common ore) Color: Black, metallic Streak: Black Hardness: 5.5–6.5 Density: Very heavy Special property: Strongly magnetic
Magnetite has the highest iron content of any common ore but is harder to smelt than hematite because it requires higher temperatures and more reducing conditions.
Field identification:
- Hold a magnet near the rock. Magnetite is the only common mineral that is strongly attracted to a magnet at room temperature.
- Black sand in stream beds is often magnetite — drag a magnet through the sand and collect what sticks.
- Streak is black (distinguishing it from hematite’s red streak).
Magnetite sand (also called ironsand or black sand) is an excellent ore source for small-scale smelting. It requires no crushing — it is already finely divided. Collect it from beaches and stream beds using a magnet.
Limonite / Goethite (FeOOH)
Iron content: 35–55% Color: Yellow-brown, orange, rusty brown Streak: Yellow-brown to ochre Hardness: 4–5.5 Density: Moderate to heavy
Limonite is not a single mineral but a mixture of hydrated iron oxides. It forms wherever iron-bearing water evaporates or where iron minerals weather in wet conditions. It is the most commonly encountered iron ore in temperate climates.
Field identification:
- Yellow-brown to rust color, often with a rough, earthy texture
- Streak is distinctly yellow-brown (not red like hematite)
- Often found as crusts, nodules, or concretions in clay soils
- Frequently associated with springs, seeps, and wetland margins
Moisture Content
Limonite contains chemically bound water. It must be roasted (calcined) before smelting to drive off this water. Heat the ore in a fire to dull red heat for several hours before charging it into the smelting furnace. Failure to do this wastes fuel and produces poor results.
Bog Iron (Limonite variant)
Iron content: 30–50% Color: Rust-brown, orange-red Form: Soft, porous nodules and crusts in wetland soils Renewable: Yes — bog iron deposits regenerate over 20–30 year cycles
Bog iron is the ore that launched the Viking Age. It forms when dissolved iron in groundwater is oxidized by bacteria at the surface of bogs, marshes, and wetlands. The resulting iron hydroxide accumulates as nodules in the top 30–60 cm of peat soil.
Field identification:
- Look in bogs, marshes, and areas where springs emerge into flat, wet ground
- Probe the soil with a stick — bog iron nodules are hard lumps in soft peat
- Surface water may show an oily, rainbow-colored iron film (iron bacteria)
- Rust-red staining in stream beds downstream of wetlands
Collection:
- Cut peat blocks and break apart to extract nodules
- Wash nodules clean of organic material
- Roast thoroughly before smelting — bog iron contains significant water and organics
- Allow the bog to rest for 20–30 years; the iron deposit will regenerate
Siderite (FeCO₃)
Iron content: 35–48% Color: Grey-brown, yellow-brown Streak: White to pale yellow Hardness: 3.5–4.5 Density: Moderate
Siderite is iron carbonate, commonly found in sedimentary formations, coal measures, and clay ironstone nodules. Historically important in early British iron production.
Field identification:
- Pale streak distinguishes it from other iron ores
- Often found as rounded nodules in shale or clay layers
- Effervesces weakly in strong acid (vinegar may produce faint fizzing)
- Turns magnetic when roasted — this is a confirming test
Processing: Must be roasted before smelting to drive off CO₂. Roasting converts siderite to magnetite or hematite, which smelt normally.
Field Testing Methods
The Streak Test
This is the most useful field test for iron ore identification:
- Find an unglazed ceramic surface — the bottom of a pottery shard, a broken tile, or a piece of porcelain
- Drag the specimen firmly across the surface
- Observe the color of the powder streak left behind
| Streak Color | Likely Mineral |
|---|---|
| Cherry red / reddish-brown | Hematite |
| Black | Magnetite |
| Yellow-brown / ochre | Limonite / goethite |
| White / pale yellow | Siderite |
The Weight Test
Iron ore is significantly denser than most common rocks. Compare the heft of a suspected ore sample against a similar-sized piece of ordinary rock (sandstone, limestone, granite). Iron ore should feel distinctly heavier.
| Material | Density (g/cm³) |
|---|---|
| Sandstone | 2.0–2.6 |
| Limestone | 2.3–2.7 |
| Granite | 2.6–2.8 |
| Limonite | 2.9–4.3 |
| Hematite | 4.9–5.3 |
| Magnetite | 5.1–5.2 |
The Magnet Test
Pass a magnet (or a magnetized needle) near the sample:
- Strong attraction: Magnetite
- Weak attraction: Some hematite varieties, roasted siderite
- No attraction: Limonite, unroasted siderite, most hematite
The Roasting Test
If uncertain about a sample, roast it:
- Heat the sample in a fire to glowing red for 1–2 hours
- Let it cool
- Test with a magnet — many iron minerals become magnetic after roasting
- Re-do the streak test — roasted ore often shows clearer color
Ore Preparation for Smelting
Before charging ore into a furnace, it must be prepared:
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Crush to walnut-sized pieces (1–3 cm). Use a hammer on a flat stone anvil. Uniform size ensures even reduction in the furnace.
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Wash crushed ore in running water or a bucket. Swirl and pour off light material (clay, organic matter). Iron minerals will sink.
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Roast (essential for limonite, bog iron, and siderite; beneficial for all ores). Build a fire on a bed of ore, or layer ore and firewood alternately. Burn for 4–8 hours. Roasting drives off moisture, burns organics, and converts carbonates to oxides. Roasted ore is easier to crush further and smelts more efficiently.
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Grade your ore. Not all pieces from a deposit are equal. The heaviest, most colorful pieces have the highest iron content. Discard light, pale pieces — they are mostly gangue (waste rock) and will consume fuel without producing iron.
Charcoal-to-Ore Ratio
As a starting point, use a 1:1 ratio by weight of charcoal to prepared ore. High-grade hematite may work at 0.8:1. Low-grade bog iron may need 1.5:1. Record your ratios and results — optimizing this ratio is one of the most impactful improvements you can make to your smelting yield.