These are commonly dyed to produce bright colours that do not occur in nature. So if you see some fuchsia chalcedony or grass green pearls, you’ll know they’ve been tinted. It’s not necessarily a bad thing in that it gives us more choices, but artificially introduced colours usually fade in sunlight and over time.
Rarely flawless in nature, almost all have been oiled for the reduction or sealing of internal fractures and for improved clarity.
Ruby, sapphire, tanzanite, citrine, aquamarine, amethyst, zircon
These are usually heated to enhance colour, clarity and brightness. Ruby and sapphire are made of the same material – corundum – but while sapphires come in all kinds of colours, only the red ones are recognised as rubies. Flawless, lab-grown corundum is also available.
Topaz (and some tourmalines)
Topaz can be found naturally occurring in a range of colours, with the reddish-pink and golden yellow being most rare and valuable. A lot of colourless or pale topaz is often irradiated and heated to produce better or new colours – blue is a common example, with a few different shades around.
Turquoise can be very fragile and porous, and may be stabilised with bonding agents to prevent discolouration and absorption of foreign substances. The range of colours from sky blue to green reflects the proportions of iron and copper within the mineral. Do note that a lot of howlite and magnesite are often dyed and sold as “turquoise”.
This is a man-made material, grown in a lab to match the appearance, chemical structure and properties as quartz that is mined from the earth – but without the flaws. Again, this allows colours not typically found in nature to be produced.