Possible Clues:

• Solid that resolves light into separate colours
• Light-diffracting glass
• Pair with a system to produce crystal
• Color-full device?
• Type of polyhedron
• Thing on the cover of 'Dark Side of the Moon'
• Glass shape splitting white light into a spectrum
• Triangular optical device
• Transparent solid for dispersing light
• Disperser or reflector of light