Question

Classify this rock by using the drop-down menus to describe the size, shape, and pattern of its grains.

Grain size:

Grain shape:

Grain pattern:

Answer 1

I don’t see an image. Please upload a clear photo of the rock (one close-up and one showing whole hand-sample) or describe it. Meanwhile, here are the typical drop-down options and quick instructions so you can pick the right answers when you have the photo:

Grain size (choose one)
- Coarse-grained (grains > ~2 mm; individual crystals/grains easily seen with naked eye) — e.g., granite.
- Medium-grained (~1/16 mm to 2 mm; grains visible but smaller) — e.g., some sandstones, diorite.
- Fine-grained (< ~1/16 mm; grains not easily seen without hand lens) — e.g., basalt, siltstone.
- Very fine / glassy (no visible grains; glassy texture or conchoidal fracture) — e.g., obsidian.
- Porphyritic (mixture of large crystals in a finer matrix — two distinct grain sizes)

How to tell: look at the surface at normal reading distance and with a hand lens. If individual crystals/grains stand out, size them roughly against a millimeter scale or compare to a grain of sand/fine salt.

Grain shape (choose one)
- Rounded (smooth, well-rounded grains — typical of transported sediment like mature sandstone)
- Subrounded (some rounding but edges still noticeable)
- Angular (sharp edges, little transport — common in breccia)
- Euhedral/crystalline (well-formed crystal faces; interlocking crystals typical of igneous or metamorphic rocks)
- Irregular/interlocking (crystals with no obvious faces, fit together like a jigsaw — common in crystalline igneous/metamorphic)

How to tell: look at individual grains/crystals. Rounded vs angular indicates transport. Euhedral faces indicate slow crystal growth.

Grain pattern/arrangement (choose one)
- Banded/Layered (distinct layers or bands) — typical of foliated metamorphic rocks or sedimentary bedding.
- Foliated/Schistose (alignment of platy minerals producing a sheen or split planes)
- Massive (no obvious pattern; homogeneous)
- Clastic/grain-supported (discrete grains cemented together, visible boundaries)
- Porphyritic (large phenocrysts in fine groundmass)
- Vesicular/porous (holes/bubbles present)
- Crystalline/interlocking (intergrown crystals forming a mosaic)

How to tell: look for layers, alignment of minerals, or a homogeneous mosaic. Presence of rounded sand grains → clastic. Parallel mineral alignment → foliated. Large crystals in a fine matrix → porphyritic.

If you upload the photo I’ll pick the three options for you.