Identifying Minerals by Streak Color

## Why Streak Matters

Streak — the color of a mineral's powder — is far more reliable than the mineral's surface color. A mineral may appear in many colors, but its streak is nearly constant. Test by rubbing the mineral firmly across an unglazed porcelain tile (Mohs ~7).

**Limitation**: Minerals harder than the streak plate (Mohs 7+) will scratch the plate rather than leaving a streak. For these minerals, crush a small fragment with a hammer to see the powder color.

## Streak Color Reference

### Red to Brown Streaks

| Streak | Mineral | Formula | Mohs | Surface Color |
|--------|---------|---------|------|---------------|
| Cherry red | Cinnabar | HgS | 2–2.5 | Red to brownish |
| Red-brown | Hematite | Fe₂O₃ | 5–6.5 | Steel gray to black! |
| Orange-red | Realgar | As₄S₄ | 1.5–2 | Orange-red |
| Yellow-brown | Goethite | FeO(OH) | 5–5.5 | Brown to black |
| Brown | Limonite | FeO(OH)·nH₂O | 4–5.5 | Yellow-brown |
| Reddish-brown | Cuprite | Cu₂O | 3.5–4 | Dark red |

### Black Streaks

| Mineral | Formula | Mohs | Surface Color | Diagnostic |
|---------|---------|------|---------------|------------|
| Magnetite | Fe₃O₄ | 5.5–6.5 | Black | Magnetic |
| Ilmenite | FeTiO₃ | 5–6 | Black | Weakly magnetic |
| Chromite | FeCr₂O₄ | 5.5 | Black | Brown streak tint |
| Uraninite | UO₂ | 5–6 | Black | Radioactive! |
| Manganese oxides | MnO₂ (pyrolusite) | 2–6 | Black | Soils hands |

### Green Streaks

| Mineral | Formula | Mohs | Surface Color |
|---------|---------|------|---------------|
| Malachite | Cu₂(CO₃)(OH)₂ | 3.5–4 | Green |
| Chlorite | (Mg,Fe)₃(Si,Al)₄O₁₀(OH)₂ | 2–2.5 | Green |
| Olivenite | Cu₂(AsO₄)(OH) | 3 | Olive green |
| Actinolite | Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂ | 5–6 | Green |

### Greenish-Black Streaks

| Mineral | Formula | Mohs | Surface Color |
|---------|---------|------|---------------|
| Pyrite | FeS₂ | 6–6.5 | Brass yellow |
| Chalcopyrite | CuFeS₂ | 3.5–4 | Brass yellow (darker) |
| Augite | (Ca,Na)(Mg,Fe,Al)(Si,Al)₂O₆ | 5–6 | Dark green-black |

### Yellow Streaks

| Mineral | Formula | Mohs | Surface Color |
|---------|---------|------|---------------|
| Sulfur | S | 1.5–2.5 | Yellow |
| Orpiment | As₂S₃ | 1.5–2 | Lemon yellow |
| Gold | Au | 2.5–3 | Gold |
| Greenockite | CdS | 3–3.5 | Yellow-orange |

### White/Colorless Streaks

Most silicates, carbonates, and phosphates produce white streaks regardless of body color. When streak is white, rely on hardness, cleavage, and other tests.

## The Hematite Surprise

Hematite is the classic example of why streak trumps color. Surface color ranges from steel gray (specular hematite) to black (massive), yet the streak is always diagnostic **red-brown**. This single test instantly identifies hematite.