Rough Looks Nothing Like the Polished Photos
Every gemstone photograph you have seen shows a cut, polished stone under controlled lighting. Raw gem material is dull, often coated, frequently fractured, and attached to whatever rock it grew in.
That gap is why so much rough gem material is thrown away as ordinary rock. A rough ruby crystal looks like a lump of dull red corundum, because that is exactly what it is. A rough sapphire is a barrel-shaped lump that looks like a worn pebble.
The good news is that uncut material has something a cut stone has lost: crystal form. The shape the mineral grew in is itself a diagnostic property, and it survives in rough where it would never survive faceting.
Crystal Form Is the Biggest Clue
The shape of the rough narrows the field faster than anything else you can observe on an uncut stone.
- Hexagonal prisms with flat ends: quartz, beryl (emerald and aquamarine), corundum (ruby and sapphire), tourmaline.
- Barrel-shaped with no sharp faces: corundum that has been rolled in a river, which is how most alluvial ruby and sapphire is found.
- Flat, tabular plates: mica, molybdenite, some corundum twins.
- Long prisms with lengthwise striations: tourmaline, which also shows rounded triangular cross-sections.
- Octahedra — two pyramids base to base: diamond, spinel, magnetite.
- Dodecahedra — twelve diamond-shaped faces: garnet, which is almost never found as anything else.
- Massive with no visible form: turquoise, malachite, jade, opal, chrysoprase.
Surface Texture and the Host Rock
Uncut gem material carries a weathered skin, and reading it is a genuine field skill. Corundum rough often has a rough, pitted, sometimes frosted surface. Beryl weathers to a dull, slightly mottled green or blue. Garnet survives as clean, sharp dodecahedra because it is hard and has no cleavage.
The host rock matters just as much. Emeralds come from veins in schist and in black shale, usually with quartz, calcite and pyrite nearby. Aquamarine and tourmaline come from pegmatites, so look for big crystals of feldspar and mica in the same rock. Peridot comes from basalt and from peridotite, and garnet comes from schist and gneiss.
Locality does a lot of work here. Knowing which gems occur in the rock you are standing on removes most of the candidates before you pick anything up.
The Tests That Work on Rough
Hardness still works, and it works better on rough than on cut stones because you have a broken face to test. Test on a fresh fracture, not the weathered surface.
Streak still works. Corundum leaves a white streak, which is how you separate it from red garnet and red spinel — both of which also streak white, so hardness does the rest. Ruby is 9 and garnet is 6.5 to 7.5.
Specific gravity works and is underused. Weigh a rough stone in your hand against a known one of similar size. Corundum at 4.0 is noticeably heavy. Beryl at 2.7 is noticeably light. Garnet at 3.5 to 4.3 sits in between.
| Rough gem | Crystal form | Hardness | Specific gravity |
|---|---|---|---|
| Corundum (ruby, sapphire) | Hexagonal prisms, or barrel-shaped if alluvial | 9 | 4.0 |
| Beryl (emerald, aquamarine) | Long hexagonal prisms, often with flat ends | 7.5–8 | 2.7 |
| Tourmaline | Long striated prisms, rounded triangular section | 7–7.5 | 3.0–3.25 |
| Garnet | Dodecahedra, almost always | 6.5–7.5 | 3.5–4.3 |
| Peridot | Short prisms and granular masses in basalt | 6.5–7 | 3.3 |
| Spinel | Octahedra, often twinned into flat triangles | 8 | 3.6 |
| Quartz | Hexagonal prisms with pyramidal ends | 7 | 2.65 |
Rough Gem or Just Rock?
Most of what people hope is rough gem material turns out to be something else. A few common cases are worth knowing.
Dull red lumps are far more often iron-stained quartzite or jasper than ruby. Green crystals in schist are more often epidote or actinolite than emerald. Clear hexagonal prisms are quartz far more often than they are beryl, and the giveaway is that quartz has pyramidal terminations while beryl ends in a flat plane.
The reliable discipline is to test rather than hope. Hardness on a fresh face, streak on a tile, and weight in the hand will tell you in two minutes whether the stone is worth pursuing further.
The Bottom Line
Raw gem material is identified by crystal form, surface texture, host rock and locality, backed by hardness, streak and specific gravity. Rough keeps the crystal shape that faceting destroys, so use it.
Corundum is hard and heavy with a white streak. Beryl is hard and light. Tourmaline is striated with a triangular section. Garnet is almost always a dodecahedron. Quartz is hexagonal with pyramidal ends.
For related guides, see raw gem identification and rough ruby identifier. Gemstone identification app covers what tools can and cannot do, beach gemstones identification covers alluvial material, and precious stone identification covers the valuable end.
Frequently Asked Questions
Start with crystal form, because uncut material still shows the shape it grew in. Then test hardness on a fresh fracture, check the streak, and compare the weight in your hand against a known stone of similar size.