Amazonite and the feldspars

Amazonite is the blue-green variety of microcline, a potassium feldspar, and it sits at roughly the same hardness as nephrite. It behaves very differently in an object, because feldspars have cleavage — two directions of internal weakness along which the crystal splits preferentially. Hardness and cleavage are independent properties, and the second one is what decides how a feldspar tool fails.

The feldspar group

The feldspars are the most abundant mineral family in the Earth’s crust — aluminium silicates with potassium, sodium or calcium, forming two series. Microcline and orthoclase are the potassium feldspars; the plagioclase series runs from sodium-rich albite to calcium-rich anorthite.

Amazonite is microcline coloured blue-green to green. The colour is generally attributed to trace lead in the structure, and it is often accompanied by white streaks or patches: these are lamellae of albite intergrown with the microcline, a texture called perthitic, and they are a natural growth feature rather than a flaw or an inclusion. A patchy white-on-green appearance is characteristic of the material rather than a sign of poor quality.

Hardness is about 6 to 6.5, comparable to nephrite. Specific gravity is around 2.55 to 2.63, appreciably lower than either jade, so an amazonite piece feels lighter than a nephrite one of the same size and much lighter than jadeite.

Other members of the group show optical effects worth knowing because they get mistaken for treatments: moonstone shows adularescence, a floating blue-white sheen from fine intergrowth layers, and labradorite shows labradorescence, strong spectral flashes from the same class of internal structure. Both are structural colour — no pigment, no coating, and frequently assumed to be one or the other.

Cleavage, and why it matters more than hardness

Cleavage is the tendency of a crystal to split along specific planes where the bonding is weaker. It is a property of the atomic structure, it is directional, and it has no relationship to hardness. Feldspars have two good cleavage directions meeting at close to a right angle — this is a defining characteristic of the group and the reason broken feldspar shows flat, step-like reflective faces rather than curved fracture surfaces.

The consequence in a thin, flat object is direct. A crack starting anywhere near a cleavage plane will follow it, and it needs less energy to do so than to break across the material. So a feldspar tool can be respectably hard, resist scratching perfectly well, and still split cleanly under an impact that a tougher material would shrug off.

This is the same lesson as hardness versus toughness with a third variable added. Nephrite has no cleavage and a felted fibrous structure, which is the best possible combination for fracture resistance. Quartz has no cleavage but is a brittle single-phase material. Feldspar has cleavage, and that puts it at the fragile end of the range at a hardness where you would not expect it.

Amazonite also frequently contains internal fractures from its geological history, and these are visible as reflective veils in transmitted light — a place a crack is already waiting.

What you can establish

WHAT YOU CAN TELL — a blue-green stone tool

  · Flat, step-like reflective break surfaces
      → cleavage. Feldspar family, not quartz or jade
  · White streaky patches within the green
      → perthitic albite lamellae; natural texture
  · Steel point does not mark it easily
      → hardness above about 6
  · Lighter than expected for a hard green stone
      → lower density than either jade
  · Sharp-edged veils in transmitted light
      → internal fractures, present from the start
  · Microcline versus another feldspar, or amazonite
      versus dyed material → NOT DETERMINABLE at
      home. Needs refractive index or spectroscopy

The cleavage observation is the genuinely diagnostic one, and it is available only on a piece that has already chipped. A flat, mirror-like break face that catches light as a single plane, sometimes in little steps, is not something an aggregate or a glass produces — nephrite, serpentine and carbonate rock break grainily and glass breaks in curves.

The names it travels under

“Amazon jade” and “Colorado jade” are the trade names most often applied to amazonite, and they are the familiar place-plus-gem-name construction doing exactly what it usually does. Amazonite is not jade in any gemmological sense; it is not even in a related mineral group.

The name amazonite itself refers to the Amazon river, and the association appears to be geographically mistaken — the material was not sourced there in any significant way. A trade name can be wrong about a place and still become the accepted mineralogical name, which is a useful reminder that the -ite ending guarantees acceptance into the nomenclature rather than accuracy about anything else.

Dyed material is a genuine issue in this colour range. Pale feldspar, howlite and various porous white stones dyed blue-green are all in the market, and dyed howlite in particular is a well-known stand-in for turquoise using the same mechanism.

Other blue-green stones, by hardness

The blue-green part of the spectrum is unusually crowded, and hardness separates most of it. Turquoise is around 5 to 6 and often porous, and much of what is sold as it is stabilised with polymer or reconstituted from powder. Chrysocolla is soft, frequently in the range of 2 to 4, and is sometimes found intergrown with quartz, which raises the effective hardness of the mixed material considerably. Larimar, a blue variety of pectolite, is around 4.5 to 5. Howlite is about 3.5 and white in its natural state, which is precisely why it takes dye so willingly.

Amazonite sits above all of them at about 6 to 6.5. So a blue-green stone that a steel point does not readily mark is in a small group, and one that marks easily is in a large one. That is a coarse observation and it does more work here than in most colour ranges.

The limit

You can bracket a blue-green stone by hardness, place it in the feldspar family if it has already broken along a cleavage plane, and recognise perthitic texture as natural. That is a decent amount for observation without instruments.

You cannot confirm the species. Distinguishing microcline from orthoclase, or amazonite from a dyed feldspar of another kind, needs refractive index measurement and probably spectroscopy — the instruments a gemmologist uses rather than anything in a kitchen. And the most consequential property here, cleavage, is one you can only verify by discovering it, which is not a test so much as an outcome.