fix(content): apply audit corrections to learn YAMLs

docs/learn/equipment/other-tools.yaml

@@ -203,7 +203,7 @@ sections:
             - ["Water (baseline)", "1.00", "None (all gems sink)", "All gemstones", "Safe"]
             - ["Toluene", "0.87", "Amber (SG 0.96–1.10)", "All other gems", "Toxic fumes — use fume hood"]
             - ["Bromoform", "2.89", "Quartz, feldspar, beryl, opal", "Tourmaline, diamond, corundum", "Toxic — use gloves"]
-            - ["Methylene iodide (pure)", "3.32", "Quartz, beryl, tourmaline", "Diamond, corundum, spinel, topaz", "Very toxic — gloves + fume hood"]
+            - ["Methylene iodide (pure)", "3.33", "Quartz, beryl, tourmaline", "Diamond, corundum, spinel, topaz", "Very toxic — gloves + fume hood"]
             - ["MI + toluene (diluted)", "3.06", "Quartz, beryl, tourmaline hovers", "Diamond, corundum, topaz", "Toxic — gloves + fume hood"]
             - ["Clerici solution", "4.25", "Diamond, corundum, topaz, spinel", "Zircon, cassiterite", "Toxic and corrosive"]
 
@@ -214,7 +214,7 @@ sections:
             - Gem
             - SG
             - Bromoform (2.89)
-            - MI (3.32)
+            - MI (3.33)
             - Clerici (4.25)
           rows:
             - ["Amber", "1.08", "Floats", "Floats", "Floats"]

docs/learn/equipment/polariscope.yaml

@@ -54,7 +54,7 @@ sections:
       rows:
         - ["Remains dark during rotation", "Isotropic (cubic system or amorphous)", "Diamond, spinel, garnet, glass"]
         - ["Light and dark every 90° (4× blink)", "Anisotropic (crystalline, non-cubic)", "Quartz, tourmaline, corundum, beryl"]
-        - ["Remains light (never dark)", "Aggregate or microcrystalline material", "Jade (jadeite/nephrite), chalcedony"]
+        - ["Remains light (never dark)", "Aggregate or microcrystalline material — but note: some types of glass also remain light throughout rotation; confirm with magnification or RI (Source: Gem-A D6-23)", "Jade (jadeite/nephrite), chalcedony; also some glass"]
         - ["Patchy light/dark (tabby extinction)", "ADR or strain birefringence", "Strained garnet, synthetic spinel"]
 
   - title: Anomalous Double Refraction (ADR)

docs/learn/equipment/refractometer.yaml

@@ -34,8 +34,8 @@ sections:
         content: |
           A standard gemmological refractometer includes:
 
-          - **High-RI glass hemisphere**: Typically RI ~1.86
-          - **Contact liquid** (RI ~1.81): Bridges the gap between gem and glass
+          - **High-RI glass hemisphere**: Typically RI ~1.86; the dense glass sets the upper limit of the measurement scale (~1.81)
+          - **Contact liquid** (di-iodomethane / methylene iodide, RI approximately 1.74): Bridges the gap between gem and glass (Source: Gem-A D6-26)
           - **Scale**: Range typically 1.35–1.81
           - **Light source**: Monochromatic sodium yellow (589nm) for accurate readings
           - **Eyepiece**: For viewing the scale and shadow edge
@@ -177,7 +177,7 @@ sections:
         - Possible Cause
         - Solution
       rows:
-        - ["No reading visible", "RI above contact liquid limit (>1.81)", "Use SG or other tests; stone may be diamond, zircon, etc."]
+        - ["No reading visible", "RI above the hemisphere glass limit (>1.81); contact liquid RI is ~1.74", "Use SG or other tests; stone may be diamond, zircon, etc."]
         - ["Fuzzy edge", "Birefringent stone, dirty contact, poor polish", "Clean surface; rotate stone; check polish"]
         - ["Moving readings", "High birefringence", "Rotate and record both extreme values"]
         - ["Very faint reading", "Poor lighting or positioning", "Improve lighting; reposition stone"]
@@ -188,7 +188,7 @@ sections:
       type: warning
       title: Handling Contact Liquid
       text: |
-        Standard contact liquid (methylene iodide, RI ~1.74-1.81) requires careful handling:
+        Standard contact liquid (di-iodomethane / methylene iodide, RI approximately 1.74) requires careful handling. Note: the 1.81 upper limit refers to the dense glass hemisphere, not the contact liquid itself.
 
         - **Toxic**: Avoid skin contact and inhalation
         - **Light sensitive**: Store in dark bottles

docs/learn/equipment/spectroscope.yaml

@@ -90,7 +90,7 @@ sections:
         - ["Blue sapphire", "Fe bands", "450, 460, 470 (three-band)"]
         - ["Almandine garnet", "Fe bands", "505, 520, 575 (diagnostic trio)"]
         - ["Zircon (high)", "Uranium lines", "Many fine lines throughout spectrum"]
-        - ["Demantoid garnet", "Cr line", "443 (horse line; diagnostic)"]
+        - ["Demantoid garnet", "Fe³⁺ doublet (438/444 nm; present in all andradite regardless of Cr content)", "443 (horse line; diagnostic — Fe³⁺, not Cr; Cr causes green colour via 618 nm and ~700 nm absorption)"]
         - ["Peridot", "Fe bands", "493, 473, 453 (three evenly spaced)"]
         - ["Yellow sapphire", "Fe bands", "450 band region"]
         - ["Yellow apatite", "Rare earth lines", "Multiple fine lines in yellow-green"]

docs/learn/fundamentals/chemical-properties.yaml

@@ -78,8 +78,13 @@ sections:
           - **Chrome tourmaline**: Green tourmaline
           - **Tsavorite**: Green grossular garnet
 
-          Chromium typically produces red in oxide minerals and green in silicates,
-          though the host mineral structure affects the exact colour.
+          The colour Cr³⁺ imparts depends on the crystal-field environment: shorter
+          Cr–O bond distances and stronger crystal-field splitting (as in ruby, Cr in
+          corundum lattice) shift absorption toward red transmission; longer bonds and
+          weaker fields (as in emerald, Cr in beryl lattice) shift toward green
+          transmission. The simple oxide-vs-silicate rule fails — red spinel is an
+          oxide, yet the absorption pattern matches the field environment, not the
+          anion class. (Source: Gem-A D7-24.)
 
       - title: Iron (Fe²⁺ and Fe³⁺)
         content: |

docs/learn/fundamentals/crystal-systems.yaml

@@ -121,14 +121,14 @@ sections:
       - title: Orthorhombic
         system: orthorhombic
         cdl: "orthorhombic[mmm]:{110}@1.0+{010}@1.2+{001}@0.8"
-        content: Three twofold axes or two mirror planes.
+        content: Three two-fold axes (or three two-fold axes plus three mirror planes for the holohedral class).
         items:
           - name: Axes
             value: "a ≠ b ≠ c"
           - name: Angles
             value: "α = β = γ = 90°"
           - name: Minimum Symmetry
-            value: "3 twofold axes or 2 planes"
+            value: "three two-fold axes (or three two-fold axes plus three mirror planes for the holohedral class)"
           - name: Point Groups
             value: "mmm, 222, mm2"
           - name: Example Gems

docs/learn/fundamentals/crystallography-advanced.yaml

@@ -163,7 +163,7 @@ sections:
       text: |
         Twinning affects optical properties and can complicate identification:
 
-        - Polysynthetic twinning causes labradorescence in labradorite
+        - Bøggild exsolution lamellae (not mechanical twinning) cause labradorescence in labradorite
         - Brazil twinning in quartz creates optical irregularities
         - Twin planes can be mistaken for cleavage
         - Twinning may cause anomalous optical behaviour under polariscope
@@ -259,9 +259,22 @@ sections:
             - ["Hexagonal", "6/mmm", "Beryl"]
             - ["Hexagonal", "6mm", "Wurtzite"]
             - ["Hexagonal", "6/m", "Apatite"]
-            - ["Trigonal", "-3m", "Corundum, tourmaline"]
+            - ["Trigonal", "-3m", "Corundum"]
             - ["Trigonal", "32", "Quartz"]
-            - ["Trigonal", "3m", "Dioptase"]
+            - ["Trigonal", "3m", "Tourmaline, dioptase"]
+
+  - title: Acentric Point Groups — Tourmaline vs Corundum
+    callout:
+      type: info
+      title: Tourmaline is Acentric (3m); Corundum is Centrosymmetric (-3m)
+      text: |
+        Tourmaline is acentric (point group 3m), which is why it exhibits
+        pyroelectricity and piezoelectricity. Corundum (-3m) is centrosymmetric
+        and therefore neither pyroelectric nor piezoelectric. Although both
+        minerals belong to the trigonal system, the presence or absence of a
+        centre of symmetry controls these physical effects.
+
+        Source: Gem-A D3 crystal systems; standard crystallography references.
 
   - title: Crystal Growth
     content: |

docs/learn/identification/synthetics.yaml

@@ -183,7 +183,7 @@ sections:
       rows:
         - ["1902", "Verneuil (flame fusion) process", "First commercial synthetic ruby"]
         - ["1917", "Czochralski pulling method", "Single crystals from melt"]
-        - ["1928", "Flux-grown emerald (IG Farben)", "First synthetic emerald"]
+        - ["c. 1935", "Flux-grown emerald (Espig / IG Farben)", "Earliest reliably-sourced experimental flux emerald. Note: some references give 1928; Gem-A Journal of Gemmology (Zerfass) puts general availability in the 1930s."]
         - ["1953", "HPHT diamond (GE)", "First reproducible synthetic diamond"]
         - ["1963", "Chatham flux-grown ruby", "Commercial flux synthetics"]
         - ["1965", "Linde star sapphire", "Synthetic star corundum"]
@@ -221,7 +221,7 @@ sections:
 
           - **Doubling**: Strong birefringence shows facet doubling under magnification
           - **Electrical conductivity**: Moissanite conducts; diamond doesn't (except Type IIb)
-          - **Specific gravity**: Lower than diamond (floats in 3.32 heavy liquid)
+          - **Specific gravity**: Lower than diamond (floats in 3.33 MI heavy liquid)
           - **Needle-like inclusions**: White, parallel needles common
           - **Thermal-electrical testers**: Dual-function testers identify moissanite
 

docs/learn/origin/burma/ruby.yaml

@@ -77,7 +77,7 @@ sections:
     content: |
       Myanmar's other major ruby source:
 
-      - **Location**: Shan State, discovered 1990s
+      - **Location**: Shan State, discovered 1992; first stones appeared in Bangkok mid-1992 (Hughes; Peretti et al. G&G Spring 1995)
       - **Character**: Often dark cores with saturated edges
       - **Treatment**: Most material heat-treated
       - **Volume**: Major production; lower quality average

docs/learn/origin/kashmir.yaml

@@ -82,7 +82,7 @@ sections:
       rows:
         - ["Tourmaline crystals", "Black prismatic crystals (schorl); diagnostic"]
         - ["Zircon with halos", "Zircon crystals with tension fractures"]
-        - ["Pargasite", "Amphibole needles"]
+        - ["Pargasite", "Calcium amphibole; occurs as needles/prisms — diagnostic of Kashmir origin (Schwieger G&G 1990; Hughes 1997; Lotus Gemology Crystal Registry)"]
         - ["Fine particulates", "Microscopic particles causing haziness"]
         - ["Colour zoning", "Irregular blue/colourless zones"]
         - ["Fingerprints", "Healed fractures with fluid remnants"]

docs/learn/phenomena/adularescence.yaml

@@ -30,10 +30,16 @@ sections:
     subsections:
       - title: Feldspar Structure
         content: |
-          Moonstone is a feldspar with alternating layers of orthoclase and albite
-          created during slow cooling (exsolution). When these layers are thin
-          enough (approaching the wavelength of light), they scatter light through
-          interference.
+          Adularescence in moonstone arises from light scattering combined with
+          thin-film interference at sub-micron alternating lamellae of orthoclase
+          and albite (Bøggild-style exsolution intergrowth, lamellae ~50–500 nm).
+          The blue/silvery sheen is wavelength-selective scattering plus partial
+          interference reinforcement. Earlier descriptions as "pure scattering"
+          (Gem-A D7 p. D7-31) or "pure thin-film interference" both capture only
+          one half of the mechanism.
+
+          Sources: Gem-A D7 p. D7-31; Gem-A Gem Hub article "Understanding
+          Moonstones and Adularescence" (Pat Daly, 2023).
 
       - title: Layer Thickness and Colour
         content: |
@@ -121,7 +127,7 @@ sections:
             - Multiple spectral colours
             - Sharp, distinct flashes
             - Appears on surface
-            - Caused by twinning interference
+            - Caused by Bøggild exsolution lamellae interference
             - Plagioclase feldspar
 
   - title: Cutting for Adularescence

docs/learn/phenomena/labradorescence.yaml

@@ -29,12 +29,17 @@ sections:
     content: |
       The cause of labradorescence:
     subsections:
-      - title: Lamellar Twinning
+      - title: Bøggild Exsolution Lamellae
         content: |
-          - Caused by light interference from repeated thin twin layers
-          - Twin lamellae form during cooling of plagioclase
+          - Caused by light interference from Bøggild exsolution lamellae
+          - The colour-causing structure is exsolution intergrowth of compositionally
+            distinct plagioclase end-members (within the Bøggild miscibility gap,
+            An ~ 47–58), not mechanical twinning. Lamellar thickness 100–300 nm
+            produces the spectral interference colours.
           - Lamellae vary in thickness, producing different colours
-          - Light interferes between twin boundaries
+          - Light interferes between exsolution layer boundaries
+
+          Source: Gem-A D7 (labradorescence section); standard mineralogy references.
 
       - title: Orientation Dependence
         content: |
@@ -102,7 +107,7 @@ sections:
             - Multiple spectral colours
             - Sharp, distinct flashes
             - Colours appear in patches
-            - Caused by lamellar twinning
+            - Caused by Bøggild exsolution lamellae
             - Plagioclase (labradorite)
             - Effect at surface level
         - title: Adularescence

docs/learn/phenomena/play-of-colour.yaml

@@ -19,12 +19,15 @@ sections:
     content: |
       Play of colour is the flashing spectral colours seen in precious opal. It is
       opal's defining characteristic and distinguishes precious opal from common
-      opal (potch). The effect results from light diffraction by regularly arranged
-      silica spheres within the opal structure.
+      opal (potch). The effect results from Bragg-style diffraction and interference
+      by regularly stacked silica spheres (150–400 nm) within the opal structure.
 
-      Unlike interference effects (adularescence, labradorescence), play of colour
-      is a diffraction phenomenon, producing pure spectral colours that shift
-      with viewing angle.
+      Play of colour is caused by diffraction and interference — the regularly stacked
+      sphere array acts as a 3D diffraction grating, while constructive/destructive
+      interference between scattered rays selects the colours seen. Viewing-angle shifts
+      arise from the geometric change in effective path difference.
+
+      Source: Gem-A D7 p. D7-32; Gem-A Gem Hub article on opal colour patterns.
 
   - title: Cause
     content: |
@@ -150,14 +153,14 @@ sections:
     subsections:
       - title: Key Differences
         content: |
-          - **Play of colour** (opal): Diffraction; pure spectral colours
+          - **Play of colour** (opal): Diffraction and interference; spectral colours
           - **Labradorescence**: Interference; colours in patches
           - **Iridescence**: Thin-film; rainbow sequence
           - **Adularescence**: Interference; single colour glow
 
       - title: Physical Cause
         content: |
-          - Opal: 3D sphere array (diffraction)
+          - Opal: 3D sphere array (diffraction and interference)
           - Labradorite: Twin lamellae (interference)
           - Iridescent gems: Surface layers (interference)
           - Moonstone: Exsolution layers (interference)

docs/learn/species/beryl.yaml

@@ -44,8 +44,8 @@ sections:
           rows:
             - ["Hardness", "7.5-8 Mohs"]
             - ["Specific gravity", "2.67-2.78"]
-            - ["Refractive index", "1.577-1.583"]
-            - ["Birefringence", "0.005-0.009"]
+            - ["Refractive index", "1.560-1.600"]
+            - ["Birefringence", "0.003-0.010"]
             - ["Optic character", "Uniaxial negative"]
             - ["Pleochroism", "Weak to distinct (varies by variety)"]
             - ["Lustre", "Vitreous"]

docs/learn/species/spinel.yaml

@@ -43,7 +43,7 @@ sections:
           rows:
             - ["Hardness", "8 Mohs"]
             - ["Specific gravity", "3.58-3.61"]
-            - ["Refractive index", "1.712-1.720"]
+            - ["Refractive index", "1.712-1.740"]
             - ["Optic character", "Singly refractive (isotropic)"]
             - ["Pleochroism", "None"]
             - ["Dispersion", "0.020 (moderate)"]