How to identify metamorphic rocks by foliation and sheen

Turn the piece under a window and watch for one orientation that behaves differently: a face that splits flat, a side with fine parallel lines, a glint that appears only from one direction, or light and dark bands all running one way. Any of those is foliation. A rock that looks the same from every side and breaks irregularly is non-foliated.

In a photo, foliation shows edge-on. Shoot the side of the piece, not the flat top, with light raking across at a low angle, then rotate ninety degrees and shoot again. Include any split face, since its quality (dull, silky, glittering or banded) places the rock on the ladder.

Slate is the lowest rung: fine-grained, splitting into flat plates with a dull sheen, in gray, black, green or purple, with no visible grains. Photograph a plate edge-on for the thin flat parting and a plate face in soft light for the sheen.

Phyllite is slate with a silky luster. Grains are still invisible, but mica has grown enough to give the split face a satin shine, often crinkled. That luster is the whole identification and shows only in angled light: tilt the face until a sheet of soft reflection sweeps across it and shoot at that angle. Front-lit, phyllite looks like slate.

Schist is where mica becomes visible. The split face glitters with flakes of muscovite or biotite, the fabric is wavy, and round red garnets, staurolite or kyanite may sit in it. A close shot a hand-width from a fresh face in side light resolves the flakes; garnets with a coin deserve their own frame.

Gneiss is the top rung: coarse, with light bands of quartz and feldspar alternating with dark bands of mica or hornblende. It is mistaken for granite because the minerals match; granite scatters them evenly, gneiss sorts them into stripes. Frame the banding running across the picture with a coin at one edge.

Both are gray, fine, and split into sheets, and the photos look alike. The safe home check is sound: tap two pieces together or tap one with a coin. Slate rings with a clear note; shale thuds. Slate plates are flatter, cleaner-edged and harder with a faint sheen, and the split often cuts across the original bedding, so color bands may cross the plate at an angle. Shale splits along the bedding, crumbles at the edges and stays matte.

Show the edge of the stack and one split face in side light, and report the ring test in the follow-up.

Quartzite is sandstone whose grains have fused: hard, white, gray, pink or tan, breaking through the grains with a glassy, slightly conchoidal surface and shedding nothing when rubbed. Marble is recrystallized limestone: white to gray, often veined, with a sugary texture of calcite grains that flash flat cleavage faces when tilted in light.

Hornfels is mudstone baked by an intrusion: dark, dense, very fine, with no fabric and a slight conchoidal break, easily taken for basalt or chert. Amphibolite is metamorphosed basalt: black to dark green, built of hornblende needles that give a faint grain even without bands, often with white plagioclase specks. Serpentinite is altered mantle rock, mottled green to black with a waxy luster and slick, polished-looking surfaces. Soapstone is mostly talc: gray-green, fingernail-soft and soapy to the touch, a safe check on a common carving stone.

Quartzite against sandstone can be settled from a good photo. Sandstone breaks around its grains, so the face is sugary with each grain standing proud; quartzite breaks through them, so the face is smoother and glassier, with old grain outlines faintly visible like ghosts. A close shot of the fracture in side light shows it.

Quartzite against marble cannot be settled by a photo when both are white and sugary. Marble often gives rhombic cleavage flashes and softer, rounded weathering; quartzite stays sharp-edged and glassy. The real difference is hardness, which is a test to describe, not to perform on a polished piece.

Foliation hides from front light and top-down shots.

Grain size and fabric rise together down the foliated rows; in the non-foliated rows, luster decides.

Site reference photos of the material, not finds from any one place.

Two questions that arrive with nearly every metamorphic upload.