How to identify igneous rocks from texture and color

Glassy texture means the melt chilled before any crystal could grow. Obsidian shows it: a broken face with smooth, curved conchoidal fracture, concentric ripples, cutting-sharp edges and a glassy luster that flares in angled light. Thin edges turn smoky translucent against daylight even when the body looks black. Photograph the broken face at a slant to a window so the ripples catch a highlight; a flat shot of a black lump shows nothing.

Vesicular texture is a froth of gas bubbles frozen in place. Pumice is pale, light enough to float, with bubble walls thinner than paper. Scoria is red-brown to black, heavier, with bigger rounder holes and thick walls. Basalt from the top of a flow carries scattered round vesicles in otherwise solid rock. A close shot a hand-width from the surface shows whether you have a stretched sponge (pumice) or round pits in a dense dark groundmass (scoria or vesicular basalt).

Aphanitic texture means crystals too small to see, so the surface looks uniform, like unglazed ceramic. Basalt (dark), andesite (medium gray, often with small white feldspar specks) and rhyolite (pink, tan or pale gray, sometimes with thin flow bands) share it. The photo clue is a fresh break with no sparkle of grains and no sandy roughness; put a coin in frame so smooth at coin scale is not mistaken for out of focus.

Phaneritic texture means every crystal is visible and interlocking, with no cement between them. Granite shows white or pink feldspar blocks, gray glassy quartz and black mica; diorite is salt-and-pepper with little quartz; gabbro is dark green-black with dull gray plagioclase. In angled light the flat cleavage faces of feldspar flash while quartz stays greasy, so shoot at a slant.

Porphyritic texture is two sizes at once: large phenocrysts in a finer groundmass, such as white feldspar laths in gray andesite or green olivine in basalt. A scale object lets a viewer measure the phenocrysts instead of guessing from the crop.

Pegmatitic texture is granite grown huge: thumb-sized or larger feldspar, books of mica with countable pages, fist-sized quartz, sometimes black tourmaline rods. A hand specimen is often one or two giant crystals and nothing else, so take an overview for size and a close shot of one crystal face.

Tuff is compacted volcanic ash: pale, light in the hand, soft enough to mark with a steel nail, with angular bits of pumice, glass and broken crystals in a dull matrix. In a photo it passes for sandstone until you notice the bits are angular, glassy and mixed in size, not rounded sand.

Color index is the share of dark minerals. Felsic rocks (granite, rhyolite, pegmatite) are white, pink or pale gray. Intermediate rocks (diorite, andesite) are salt-and-pepper gray. Mafic rocks (gabbro, basalt, scoria) are dark gray to black. Two rocks break the rule: pumice is pale foam and obsidian is black glass, yet both are usually rhyolitic, so the glassy and frothy rocks must be read by texture alone.

Weathering shifts color. Basalt goes brown or rusty orange, granite feldspar turns chalky, obsidian grows a dull gray skin. Photograph the fresh face for the color index and the weathered face as a second shot.

Grain size is the column that matters most; color narrows it; the distinctive clue is what a good photo should show.

Most igneous photos fail because only the weathered skin is shown or the light is flat. This order fixes both.

Slag, the waste from smelting, is the usual false obsidian and false scoria. Look for colors no lava has (bottle green, blue, metallic swirls), bubbles stretched in one direction, rusty metal blebs and a flat poured base. It turns up near railroads, old furnaces and fill, not on a lava field.

River-tumbled bottle glass mimics obsidian pebbles. It has uniform color with no inclusions, even thickness, and sometimes mold seams or lettering; obsidian carries tiny crystals or flow bands and a smoky translucent edge.

Broken concrete with coarse aggregate gets uploaded as granite. The giveaway is rounded gravel of mixed rock types in a gray paste; granite's crystals interlock with nothing between them.

Site reference photos of the material, not finds from any one place; use them to calibrate coarse, fine and glassy at phone distance.

Two questions that arrive with nearly every igneous upload.