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Quartz


Quartz is the second most abundant mineral in the Earth’s crust after feldspar and occurs in two main types: crystalline and polycrystalline. Crystalline quartz forms as relatively large crystals mostly visible to the unaided eye, whereas polycrystalline quartz exists as masses of tiny crystals or grains.

Characteristics

Crystalline quartz normally forms as hexagonal prisms and has a hardness score of 7 on the Mohs scale. Polycrystalline quartz scores 6 to 7. It comes in a vast range of colours, some of which we will explore below.

Crystalline quartz

 

Rock crystal

A colourless, almost pure quartz. It occurs worldwide with some crystals exceeding two tonnes in size. It is popular as an ornamental stone but is also cut as gemstones or used in objets d’art.


Amethyst

The purple variety of quartz obtains its colour from its iron content. It is often seen in jewellery but is also popular when found in geodes. Geodes are roughly spherical lumps of rock containing masses of crystals that have formed usually from minerals deposited by hydrothermal fluids seeping through the rock. A geode is pictured above.

Citrine

Orange quartz, again coloured by iron. Sometimes it can be more yellow, golden or brown and it is mostly actually heat treated amethyst.

Rose quartz

A pink quartz coloured by titanium and iron. It is often found as rock rather than as distinct crystals and can make a beautiful ornamental stone.

Polycrystalline quartz


Carnelian

Also known as cornelian, it is a bright orange or reddish quartz often seen in signet rings or set in fobs.

Chrysoprase

A beautiful green quartz coloured by nickel, sometimes mistaken as good quality jadeite when set in jewellery.

Agate

Often seen as ornamental stones cut to show off its distinct curved banding.


Tiger’s eye

This is formed where blue crocidolite asbestos is replaced by quartz which retains the fibrous appearance of asbestos. It is coloured by iron oxides and ranges in colour from shades of yellow to deep golden brown and blue.

If you have any quartz which you would like to have valued, please contact Simon Rufus, jewellery specialist at Grand Auctions, Folkestone, Kent.


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