Rock and Mineral Identification With a Phone [Field Guide]

Start with what kind of question you are asking

A rock and a mineral are different things, and they are identified differently. A mineral is a single natural substance with a fixed chemistry and crystal structure, such as quartz, calcite, or pyrite. A rock is a mixture of mineral grains, such as granite or sandstone. Most things you pick up on a trail are rocks, and identifying one means figuring out which minerals are in it, how big the grains are, and how they are arranged.

Magnification helps both jobs. At 5x to 10x you can see the grains in a rock and how they fit together. At 10x to 30x you can see crystal faces, cleavage planes, and surface luster on a single mineral. This guide walks through the properties in a sensible order and shows where a phone magnifier fits in.

Get a fresh surface

Weathered rock surfaces lie. Iron staining, lichen, and a dull rind hide what is underneath. Where it is permitted, break off a small corner with a rock hammer (wear eye protection), or find a recently broken edge. Everything below assumes you are looking at a fresh surface.

Also wet the specimen. A splash of water darkens the surface, kills glare, and makes grain boundaries and color differences much easier to see, both by eye and at 10x.

The properties, in a useful order

Luster

Luster describes how the surface reflects light: metallic like a polished metal, glassy like broken bottle glass, pearly, silky, greasy, or dull and earthy. It is one of the most reliable first observations, and magnification makes it easier because you can see the luster of a single small grain rather than the average of the whole rock. Tilt the specimen under a lamp while looking at 10x. Pyrite and mica, for example, both sparkle to the eye, but at 10x pyrite is brassy metallic and mica is a stack of thin glassy sheets.

Hardness

The Mohs scale runs from talc at 1 to diamond at 10. You do not need a reference set. A fingernail is about 2.5, a copper coin about 3, a steel nail or knife blade about 5.5, and glass about 5.5 as well. Try to scratch the mineral with each in turn. A mineral that scratches glass is harder than 5.5; one scratched by a fingernail is softer than 2.5. Check the result at 10x to 20x, for example by photographing the test spot with LoupeLens and zooming into the still image. A faint line may be metal rubbed off the nail rather than a real scratch, and the difference is obvious under magnification.

Cleavage and fracture

Cleavage is the tendency to break along flat planes. Mica splits into sheets, calcite breaks into rhombs with three cleavage directions, feldspar shows two cleavages at roughly right angles. Quartz has no cleavage and breaks in curved conchoidal fractures like glass. At 10x to 20x, cleavage shows as flat, step-like surfaces that catch the light together when you tilt the specimen. This is one of the places a magnifier earns its keep, because small cleavage faces are easy to miss.

Crystal shape

Where crystals have had room to grow, their shape is diagnostic: cubes for pyrite and halite, six-sided prisms with pointed ends for quartz, rhombs for calcite, blades for kyanite. In most rocks the crystals are crowded and irregular, but at 20x you can often find one grain that shows a face or two.

Streak

Rub the mineral on an unglazed porcelain tile and look at the color of the powder. Streak is more consistent than the mineral's body color. Hematite, for instance, can be silver or black but leaves a reddish-brown streak.

Color

Last for a reason. Quartz comes in clear, white, pink, purple, smoky gray, and yellow. Color narrows things only after the other properties have done their work.

Reading a rock at 10x

For a rock, the magnifier shows three things that lead you to the rock type.

• Grain size and shape. Interlocking angular crystals suggest an igneous or metamorphic rock. Rounded grains held in a cement suggest a sedimentary rock like sandstone.
• Which minerals. In granite you can pick out glassy gray quartz, blocky white or pink feldspar, and flaky black or silver mica. If the dark minerals dominate and there is no quartz, you are looking at something else.
• Arrangement. Layers, bands, or aligned flaky minerals point to a metamorphic rock such as schist or gneiss. Random arrangement points to igneous. Fossils or bedding point to sedimentary.

With LoupeLens, wet the specimen, hold the phone a few inches above a fresh surface, zoom to about 10x, and tap on a grain boundary to focus. Take a photo, then zoom into the still image to examine individual grains. Repeat at 20x on any grain that looks interesting. If you are in shade, the flashlight helps, but for luster and cleavage a low side light from the sun or a separate lamp works better than the flashlight, which flattens everything.

Quick reference for common minerals

Mineral	Luster	Hardness	Cleavage or fracture	Tells at 10x to 20x
Quartz	Glassy	7	No cleavage, conchoidal fracture	Curved glassy fracture surfaces
Feldspar	Glassy to pearly	6	Two cleavages near 90 degrees	Flat stepped faces, often striated
Mica	Glassy, pearly	2 to 3	One perfect cleavage	Thin flexible sheets, layered edges
Calcite	Glassy	3	Three cleavages, rhombs	Rhombic fragments; fizzes in vinegar
Pyrite	Metallic	6 to 6.5	Fracture	Brassy cubes, striated faces
Hematite	Metallic to dull	5 to 6	Fracture	Reddish-brown streak
Garnet	Glassy	6.5 to 7.5	Fracture	Rounded 12-sided crystals in schist

Photographing specimens for a club or a collection

Rock and mineral clubs and online groups are generous with identifications when the photos are useful. Shoot on a plain background in daylight, include a coin or ruler for scale, take one photo of the whole specimen and two or three at 10x to 20x of fresh surfaces, and photograph both wet and dry. Say where you found it, at least to the region, and what the surrounding rock was. Location is often half the identification.

For your own collection, a labeled photo set with location, date, and your best guess makes a good catalog, and it is far easier to search than a box of paper tags. The same approach works for coins and other small collectibles.

Limits and safety

A phone magnifier shows what a hand lens shows. It will not tell you a rock's chemistry, and many minerals look alike until tested. Some minerals are best not handled or inhaled as dust, so avoid grinding or breaking unknown specimens indoors, wash your hands after handling, and wear eye protection when using a hammer. Collect only where it is permitted, and leave protected sites as you found them. Identification gets easier with every specimen you work through, and the magnifier makes the learning faster because you are actually seeing the properties the books describe.