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Rainbow iridescence can occur on minerals just as it does with soap bubbles or oil sitting on water.
Light waves are reflected off the top and bottom of a thin surface causing the waves to be slightly out of phase with each other when reflected. The various layers and layer thicknesses determine the colour seen during this reflection. For minerals such as ammolite (gemstone case), the very thin layers are made up of aragonite.
For other minerals such as marcasite (sulphides case) and chalcopyrite (SA Copper Belt and Tasman Fold Belt cases) the effect is caused by surface tarnish. This optical property can also occur on laboratory formed minerals such as synthetic bismuth (Human-made case).
Opal: Play of colour
Beautiful play of colour occurs when silica opal spheres are uniform of size and stacked (like rows and columns of marbles) creating voids between the stacks. Light enters the surface and the voids provide an optical discontinuity where light can be reflected and diffracted into the individual wavelengths of light, similar to the colours seen in a rainbow.
The sphere/void size determines the colour seen in that colour patch; the smallest sphere produces violet through to the largest sphere producing red (Opal Foyer, Gemstone and SA Copper Belt cases).
The Rainbow flag, symbol of LGBTQIA+ Pride
The rainbow flag was first designed by activist and artist Gilbert Baker in 1978 as a symbol of unity and pride for the LGBTQIA+ community. The original flag featured eight colours, each with a specific meaning: pink for sexuality, red for life, orange for healing, yellow for sunlight, green for nature, turquoise for art, blue for harmony, and violet for spirit.
The flag was intended to show inclusivity and diversity, and it has been widely adopted as the symbol of the LGBTQIA+ movement. From its creation, the flag has gone through many variations, and it has been used as a symbol of pride and community at LGBTQIA+ rights events and parades ever since.