WEEK 12 - Optical Properties of Lustre Flashcards
What factors determine the lustre of nacre?
- Surface reflection quality
- Internal reflection quality
How do aragonite tablets affect lustre?
- Aragonite tablets reflect light like mirrors for surface reflection.
- Aragonite tablets transmit light, causing internal reflection for a warm glow.
What is the iridescent play of colours in nacre called?
Orient: The effect caused by the reflection and internal behavior of light in nacre.
How does surface relief influence orient in nacre?
Diffraction: Light waves split into colours due to grooves/ridges on the surface.
Constructive interference: At certain angles, some colours are reinforced, making them more brilliant.
How do refraction and reflection influence orient in nacre?
Aragonite crystals act as tiny prisms, refracting light into colours.
Constructive interference: Outgoing light waves bounce off crystal surfaces, enhancing the effect.
What are some uses of nacre?
Shell collectors: Remove the prismatic layer to reveal the nacreous layer.
Jewellery and inlays: Used in ornaments and musical instruments.
Buttons: Widely used in button-making.
What is “Nautilus” in art?
Photograph: “Nautilus” by Edward Weston (1927).
Significance: One of the most famous photos ever made.
Modernism: A benchmark in the history of photography.
Value: 14th most expensive photograph.
What is Ammolite?
Fossilized ammonite nacre, a rare gemstone.
Why is Ammolite Rare?
Aragonite typically dissolves or turns into calcite under normal preservational conditions, making fossilized nacre uncommon.
What is the Source of Ammolite?
From ammonites, extinct relatives of the Nautilus (squids, octopuses, and cuttlefishes).
What was the Structure of Ammonites?
Ammonites had chambered shells filled with gas and liquid for buoyancy regulation.
Where is Ammolite Found?
- Exceptionally well-preserved ammonite nacre occurs in the Cretaceous Bearpaw Shale, south of Lethbridge, Alberta.
- Approximately 70 million years old.
When was Ammolite Granted Gem Status?
1981 by the World Jewellery Confederation.
What is the Official Status of Ammolite in Alberta?
It has been the official gemstone of the Province of Alberta since 2004.
Where is Ammolite Commercially Mined?
South of Lethbridge, Alberta.
Why is the Play of Colours in Ammolite Enhanced?
- The play of colours in ammonite nacre from the Bearpaw Shale is greatly enhanced in intensity.
- This might be due to slight deformation of aragonite crystals within the nacreous layers.
How Does Modern Nautilus Compare to Ammolite in Terms of Iridescence?
- Shows subtle iridescence (outer prismatic layer removed).
- In contrast, fossil nacre from ammonites like Placenticeras (Bearpaw Shale, AB) displays bold iridescent colours.
Why is Ammolite Difficult to Work With?
- Ammolite readily splits apart along planes between aragonite sheets.
- It is quite soft and prone to scratching.
How is Ammolite Processed for Jewellery?
- Must be processed differently than most gemstones due to its fragility.
- In jewellery, there is often very little ammolite in the actual setting.
What is an Ammolite Triplet Cabochon?
- Most ammolite is mounted in triplet form.
- It consists of a thin wafer of ammolite, an opaque black backing, and a cap of scratch-resistant quartz or spinel.
How is an Ammolite Triplet Cabochon Made?
- The ammolite is ground down and polished into a thin wafer
- An opaque black backing is glued to the back of the ammolite wafer.
- A cap of scratch-resistant quartz or spinel is glued on top.
