Identification

Mineral Identification Basics: Hardness, Streak, Luster and Cleavage

Color alone can fool you. Geologists combine a few simple tests.

1. Hardness (Mohs scale)

The Mohs scale ranks scratch resistance from 1 (talc) to 10 (diamond): talc, gypsum, calcite, fluorite, apatite, orthoclase feldspar, quartz, topaz, corundum, diamond. It is relative, not absolute. Handy reference points: fingernail about 2.5, copper penny about 3, pocket knife a bit over 5, window glass 5.5, steel file 6.5.

Test both ways, and rub the mark with a finger: a true scratch stays, while powder from the softer material rubs off.

2. Streak

Streak is the color of the mineral's powder, made by dragging it across unglazed porcelain. It is more consistent than the color of the piece itself. Hematite always has a red-brown streak. A porcelain plate is about 6.5 hard, so harder minerals just scratch it.

3. Luster

Luster is how the surface reflects light: metallic, glassy (vitreous), pearly, silky, waxy, resinous or earthy. Pyrite and galena are both metallic; quartz is glassy.

4. Cleavage and fracture

Some minerals split along flat planes. Mica splits into thin sheets; feldspars break into blocks along two cleavages near right angles; galena breaks into cubes. Quartz and garnet have no cleavage and break irregularly.

Try it on New Hampshire minerals

• Mica: thin flexible sheets, about 2 to 3 hard. Mica in NH
• Feldspar: blocky, 6 to 6.5 hard. Feldspar in NH
• Beryl: six-sided crystals, 7.5 to 8 hard. Beryl in NH
• Pyrite: brassy and metallic, 6 to 6.5 hard. Pyrite in NH
• Fluorite: 4 hard, cubes. Fluorite in NH

Sources: Mineralogical Society of America, Mineral ID Key; Australian Museum, Mineral properties; Kansas Geological Survey; hardness values from Mindat.