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What Is This Black Stone? A Field Guide to Black Crystals

By Digital Towns Crystals ยท Last reviewed October 8, 2026

Short answer: black stones are easier than they look because four quick checks split them. A red-brown streak means hematite; a black streak plus a magnet pull means magnetite (or magnetic "hematite"). Striated, three-sided prisms are black tourmaline. A glassy stone with shell-shaped breaks and no crystal faces is obsidian. Even, waxy black that scratches glass is usually dyed chalcedony sold as onyx, and a light, matte stone that leaves black on paper may be shungite.

Which black stones turn up most often?

Color tells you almost nothing in this group, so identification rests on luster, weight and how the stone breaks. Six materials fill most black trays at shows and shops:

  • Black tourmaline (schorl), a boron silicate that grows as long, grooved prisms.
  • Obsidian, natural volcanic glass with no crystal structure at all.
  • Onyx, in the black trade sense a chalcedony, nearly always dyed.
  • Hematite, an iron oxide with a metallic gray-black polish and surprising weight.
  • Shungite, a carbon-rich rock from Russian Karelia.
  • Smoky quartz at its darkest, which can read as black until you shine a torch through an edge.

Black kyanite, magnetite, jet, black spinel and black-dyed agate make up most of the remainder. The point of the next section is to get from "a black stone" to one of those names in under five minutes.

Which four questions sort any black stone?

1. Is the surface metallic?

A mirror-gray, metallic shine narrows the field to iron oxides and a few look-alikes. Drag an unpolished edge across the back of a ceramic tile. Geology.com explains that all specimens of hematite produce a reddish streak and that the red streak is the most important clue, even on a silver-colored piece; its table gives hematite a specific gravity of 5.0 to 5.3. Magnetite leaves a black streak and, in Geology.com's words, is strongly magnetic, with a specific gravity of 5.2.

Then bring a fridge magnet close. Natural hematite is not magnetic, although some specimens contain enough magnetite to respond. The polished, strongly magnetic beads sold as "magnetic hematite" are another matter: the International Gem Society says material sold as "hematine" or "magnetic hematite" contains no hematite at all. A bead that snaps to a magnet is manufactured; the magnetic hematite guide explains what it is made of.

2. Are there crystal faces or grooves?

Long prisms with deep, parallel grooves running their length are schorl. Minerals.net describes it as elongated prismatic crystals that are heavily striated, many with a rounded, triangular cross-section, and lists a hardness of 7 to 7.5 and specific gravity of 3.1 to 3.2. It also calls schorl the most common form of tourmaline. Look at the broken end of a tourmaline rod and you will see a rounded triangle, not a hexagon.

Flat, flexible-looking blades that fan out like a broom are black kyanite. Geology.com notes that kyanite has a variable hardness, about 4.5 to 5 along the length of a crystal and 6.5 to 7 across it, so a knife scratches it one way and not the other.

Six-sided, glassy points that let light through at a thin edge are dark smoky quartz.

3. Does it break like glass?

Shell-shaped, curved fracture surfaces with razor edges and no faces at all mean natural or man-made glass. Geology.com's obsidian page describes obsidian as a volcanic glass with a smooth uniform texture that breaks with a conchoidal fracture, with a hardness of about 5.5. That softness separates it from tourmaline and quartz: obsidian will not scratch a steel file, while tourmaline will. The full test sequence is in the obsidian versus black tourmaline comparison.

4. Is it light, matte and soft?

A black stone that feels oddly light, looks dull rather than glassy, and leaves a gray-black mark on paper is likely shungite or jet. An article from the New Zealand Skeptics describes shungite as a black stone with a carbon content usually of 30 to 95 percent, originally discovered in Shunga in the Karelia region of Russia.

Black stone decision table

Observation Most likely Confirm
Metallic, red-brown streak, heavy Hematite Little or no magnet pull
Metallic, black streak, strong magnet pull Magnetite Octahedral crystals if any
Polished beads, strong magnet pull, uniform Manufactured "hematine" Price and perfect uniformity
Grooved prisms, triangular cross-section Black tourmaline Scratches glass
Bladed, fan-shaped crystals Black kyanite Knife scratches lengthwise only
Glassy, conchoidal fracture, no faces Obsidian, or glass Loupe for round gas bubbles
Waxy, even black, scratches glass Dyed chalcedony sold as black onyx Disclosure, cracks
Light, matte, marks paper Shungite or jet Low weight in hand
Translucent brown at thin edges, six-sided Smoky quartz Torch through an edge

The specific gravity test separates hematite (around 5) from tourmaline (3.1 to 3.2) and obsidian (2.33 to 2.42) with a kitchen scale.

How much of the black on the market is dyed?

A great deal, and in one category nearly all of it. Minerals.net's onyx entry states that black onyx is often dyed, while the banded white and black form is usually natural. The method is old: the Ganoksin article on black coloration explains that boiling in sulfuric acid reduces sugar or honey soaked into the stone to carbon, a workaround because carbon itself is insoluble in almost everything, and notes that agate so colored has long been on the market as "black onyx". With black onyx the practical question is disclosure, not whether it is dyed. A plain black onyx bead at a low price is almost certainly dyed agate, and that is normal in the trade. The dyed agate guide covers the process in more depth.

Glass is the other substitution. The gem laboratory EPI reports glass sold as obsidian in bead chains and tumbled stones that looks natural to the naked eye, with numerous round gas bubbles visible only under a magnifier. A 10x loupe on a few beads is a quick screen. The International Gem Society obsidian page adds that jet, chalcedony and schorl are the materials most often substituted or confused with obsidian, and that obsidian typically has a specific gravity of 2.33 to 2.42. The obsidian versus onyx comparison and glass imitations cover both.

Can black stones fade or change?

Most black materials are colored by elements in the formula, carbon or opaque iron oxides, and do not fade. Smoky quartz is the exception. Its color comes from natural radiation, and Geology.com notes that some rock crystal can be given the color of smoky quartz by irradiation in a laboratory, and that natural smoky quartz with a very dark color is sometimes heated to lighten its color. The irradiated crystals guide explains how to read those listings, and why crystals have color explains color centers.

In crystal traditions, black stones are associated with grounding and protection. That association says nothing about which mineral is in your hand.

Shopping black crystals with confidence

For black stones, the listing should name the mineral and show a natural or broken surface where possible, since polish hides the striations and fracture patterns that identify them. These pieces each show one of the tests above.

  • A 588 g elongated black tourmaline hunk: at 6.25 inches long, the striated, columnar structure runs the full length, which is the best single proof of schorl.
  • Black Obsidian Crystal Sphere Polished Volcanic Glass: 50 mm and 60 mm spheres (166 g and 290 g); compare the weight with a tourmaline or hematite piece to feel the difference in density.
  • Hematite: small tumbles, palm stones and an unpolished botryoidal piece; the rough piece is the one to streak-test.
  • Shungite: Russian tumbles, spheres, palm stones and small towers, showing the matte gray-black finish.
  • Black Kyanite: 2.5 to 3 inch fan-shaped blades, the shape no other black mineral copies.
  • A 60 mm smoky quartz sphere: 260 g of dark quartz that turns brown and translucent when a torch is held behind it.

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Frequently asked questions

How do you tell black tourmaline from obsidian?

Look for grooves and faces. Black tourmaline forms striated prisms with a rounded triangular cross-section and scratches glass. Obsidian is glass: no faces, curved shell-like breaks, and a hardness near 5.5, so it will not scratch hard steel.

Is black onyx a natural stone?

The base material is natural chalcedony or agate, but solid black onyx is almost always dyed, traditionally with a sugar and acid process that leaves carbon in the pores. Banded black and white onyx is usually undyed.

Why does a black stone stick to a magnet?

Magnetite is naturally magnetic, and some hematite contains enough magnetite to respond. Strongly magnetic polished beads sold as magnetic hematite are manufactured and contain no hematite. Real hematite is not magnetic and leaves a red-brown streak.

Why is a black stone leaving marks on paper?

Soft, carbon-rich materials such as shungite and graphite leave gray to black marks. Hematite leaves a red-brown mark on an unglazed tile, not a black one. Tourmaline and obsidian are too hard to mark paper.

Is very dark smoky quartz natural?

It can be. Natural smoky quartz ranges to near black (sometimes sold as morion), but colorless quartz can also be irradiated in a laboratory to produce the color. Ask the seller about treatment, and check that the color looks natural rather than uniformly dark.

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