Veins, inclusions, and internal lines

Every natural stone has internal features, and the ones you can see in a polished tool fall into two categories that matter for entirely different reasons. Some are inclusions and growth structures — foreign material or textures the stone formed with, which are identification evidence. Others are fractures, healed or open, which are structural weakness. Telling them apart is a genuinely useful skill and it needs a torch, not a laboratory.

Inclusions

An inclusion is anything enclosed within a mineral that is not the mineral itself: a crystal of another species, a pocket of fluid or gas, or a mass of fine particles.

They are the most information-dense features a stone has. Because an inclusion formed under the same conditions as its host, it records where and how the material grew, which is why gemmologists study them closely and why they are the primary route to distinguishing natural from synthetic material.

In the tool materials, the inclusions you can see are usually populations rather than individuals. The platy mica flakes of aventurine are inclusions; so are the fibrous inclusions that make rose quartz pink and cloudy; so are the white crystal clusters in snowflake obsidian. In the jadeite trade, whitish fibrous patches are described as “cotton” — a natural textural feature of the material, not damage.

The key observation about inclusions: they have volume and mineral character. Under a torch they look like objects — with shape, sometimes colour of their own, sometimes reflective faces. They are not lines.

Growth and rock structures

Distinct from inclusions, and often mistaken for cracks.

Banding is layered variation in colour or texture, common in chalcedony, agate and many sedimentary rocks. Colour zoning is angular sectors following former crystal faces — the geometric banding typical of amethyst. Perthitic lamellae are the white streaks in amazonite, one feldspar intergrown with another. Veining in serpentinite and marble is genuinely mineral-filled: fractures that opened during the rock’s history and were filled by later mineral growth, now a continuous part of the solid.

These are all part of the material’s structure. A mineral-filled vein has strength, though usually less than the surrounding rock, because the filling is a different mineral with a different hardness and a boundary on each side.

Fractures

A fracture is a break, and there are three states worth distinguishing.

Open fractures — a genuine gap, filled with air. These reflect light strongly, appearing as a bright mirror-like plane when caught at the right angle, and they are the most serious defect in a thin object. A tool with an open internal fracture across a narrow section is a tool with a decided future.

Healed fractures — a former break through which mineralising fluid passed, redepositing material and rejoining the two sides. Under magnification these often show a distinctive pattern of trapped fluid droplets in a plane, a texture gemmologists call a fingerprint. Considerably stronger than an open fracture, still a plane of relative weakness.

Filled fractures — an open fracture into which something has been introduced deliberately: resin, glass, oil, wax. The point of the exercise is to reduce visibility by putting a substance with a refractive index closer to the stone’s into the gap. This is treatment, and at the extreme it is polymer impregnation of the whole piece rather than of one crack.

The practical distinction: an open fracture flashes bright as you rotate the stone and then disappears. An inclusion or a healed structure remains visible from most angles because it scatters rather than reflects.

Reading them

WHAT YOU CAN TELL — internal features under a torch

  · Bright mirror flash appearing at one angle only
      → open fracture. Structural weakness
  · Plane of tiny droplets, visible from most angles
      → healed fracture; natural
  · Object with shape, colour or mineral faces
      → inclusion; identification evidence
  · Continuous filled line of different material
      → mineral vein; part of the rock
  · Spherical bubbles → glass or resin. Reliable
  · Whether a fracture has been resin-filled
      → NOT DETERMINABLE at home. Needs infrared
        spectroscopy

What internal features do to an object

The mechanics are simple and unforgiving. A thin plate under bending stress concentrates that stress at its narrowest section, and an internal fracture in the wrong place removes most of the material’s resistance there. This is why tools break where they break rather than where they were hit.

Two consequences follow.

Position matters more than size. A small open fracture at a narrow waist is worse than a large one in the thick body of a piece, because that is where the stress is.

Thermal change finds them first. A rapid temperature change loads the whole surface, and the existing crack is the easiest place for that load to relieve itself. A piece that cracked “spontaneously” in warm water usually had the crack already; the water only finished it.

Neither of these is advice about what to do with the object, which is not this site’s subject. They are what the features mean mechanically.

How the trade handles them

Internal features are a grading factor, so there is money in hiding them, and the methods are the ones above: filling with something optically close to the stone, waxing the surface so that surface-reaching cracks disappear, and orienting a cut so a fracture runs parallel to a face rather than across it.

There is also an honest version, which is that visible features are normal. Natural stone at these prices has fractures, veins and inclusions in it, and a completely clean, featureless, uniform piece of supposedly natural material is the anomaly rather than the prize — it is more likely to be glass or a moulded composite than exceptional stone.

The limit

You can find internal features, tell reflective open fractures from scattering inclusions, and recognise banding and veining as structure. That is a real capability and a torch in a dark room is all it takes.

What you cannot do is determine whether a fracture has been filled, or identify an inclusion’s mineral species. Both need instruments — resin detection in particular requires infrared spectroscopy, which is squarely in a gemmologist’s hands. And an absence of visible fractures is not evidence of their absence; most of the interior of an opaque stone is not visible to you at all.