Rock crystal: April’s other birthstone

Twinned double-pointed colourless and clear rock crystal called Herkimer diamond

Rock crystal is beautiful, powerful and has been used in adornment for thousands of years. It is April’s less famous, but no less beguiling, birthstone.

My birthstone for April is diamond or rock crystal. I remember finding this out when a child and being quite pleased, thinking diamonds are expensive! Somehow I thought that caché might rub off on me… Then I grew envious of those with more colourful birthstones. As an emerald fan I particularly wanted a May birthday. My zodiac birthstone for Aries is bloodstone, I’ll write more about those another time.

Rock crystal is a variety of crystalline quartz with particular clarity and transparency. It is a relatively hard gemstone with a hardness of 7 on the Mohs scale and relatively tough, making it a super stone to facet and shape.

Cornish and other ‘diamonds’

On the south Cornish coast it isn’t unusual to see rock crystal pebbles, worn smooth by the waves and tides but often included and pitted. Being granite-based and quartz-rich, all manner of quartz varieties, crystalline like amethyst, and microcrystalline chalcedonies like carnelian and banded agates, as well as jaspers, can be found. There are near-infinite varieties of quartz gemstones.

In the 16th to mid-18th centuries when knowledge of real diamonds in Europe was still basic, rock crystal had also been described as ‘bastard diamonds’ or ‘Cornish diamonds’.

Herkimer diamonds from Herkimer County, New York, USA, are rock crystal with particularly excellent clarity and usually forming in double-terminated prismatic crystals (see featured image above). Do not get confused with real rough diamond crystals which are usually octahedral rather than rock crystal’s six-sided prisms with terminations.

Pebbly beaches in south Cornwall can hide many a quartz gemstone.

How to tell the difference between diamond and rock crystal

It is relatively easy to tell a real diamond apart from rock crystal even when cut and set. You could use a diamond identifying instrument such as a thermal probe – rock crystal is a thermal insulator whereas diamond is a thermal conductor (relative to each other). If you can hold both you will notice the diamond ‘feeling cooler’ than the rock crystal and this is true of most other diamond simulants.

The best clue, in my view, without gemmological instruments, is to learn to look for diamond’s sheer brilliance, high lustre (adamantine, the brightest reflection of white light) and fire (the rainbow flashes you observe when turning the stone). Rock crystal is totally different in nature, it does not exhibit fire and its lustre is more vitreous, i.e. like the reflection of daylight you might get off window glass rather than the bright flashlight effect you get from a well-cut diamond facet.

If you are a budding gemmologist it is easier still to tell the difference between diamond and rock crystal. Diamond is much more dense than rock crystal – so a one carat diamond will be noticeably smaller than a one carat rock crystal cut identically. Crucially, diamond is isotropic or singly refractive (SR) which means light moves in the same way through the gem material in all directions. All crystalline quartzes like rock crystal are anisotropic or doubly refractive (DR) which means light splits into two different directions when it travels through the stone. You will observe this using the polariscope (be careful examining diamonds on glass stages such as the polariscope as diamonds are so much harder than glass and can easily scratch). SR gems like diamonds and spinel will remain dark on rotation through 360 degrees and DR gems like quartz will sweep from dark to light four times through 360 degrees. Don’t bother with the dichroscope as it tells you nothing about colourless gemstones.

In addition, rock crystal like other quartzes rate 7 on the Mohs scale of hardness, and diamond is exponentially harder at 10. Under a loupe, the facets of rock crystal may be more soft-looking or rounded, possibly even abraded when compared with diamond facet edges which are sharp and clear.

What to look for in rock crystal

For me excellent specimens of rock crystal, rough and cut, should have the quality of the freshest, coolest, spring water. You should feel you want to take a drink or take a deep breath of cool and clean hillside or mountain air. Inclusions such as partially-healed fissures should enhance the look of the gemstone or it should be flaw-free (cracks and dirty looking inclusions). It should take a relatively good polish without leaving scratches and pits.

When talking about rock crystal, I am also including other types of crystalline colourless quartz which are known for their visually-arresting inclusions, such as tourmalinated quartz (distinct black rods of crystalline tourmaline) and rutilated quartz (fine golden, reddish or silver hair-like crystals). Either of these types would make excellent, completely unique birthstone jewellery as would rock crystal exhibiting other kinds of inclusions such as platy metals (strawberry quartz), enhydros (containing primeval ‘water’ trapped during crystal formation).

Oval mixed cut rock crystal with striking silver rutile inclusions. The arrangement of the rutile inclusions is impressive from the pavillion (credit: Tehmina Goskar).

Recently in a local branch of the lovely store Crystals I saw a pair of faceted rock crystal earrings, pear-cut and set in a silver open-back bezel which had the effect of mirroring the facet reflections several times over. They almost formed a kaleidoscope which is unusual for a near-colourless stone. However I also observed a most subtle ‘smoke’ in them. I really wanted them. I left the shop, and then returned some days later. They were still there, and so meant to be, I captured them.

Two medieval rock crystal jewels

1. The Warminster Jewel, possibly late 9th century. Salisbury Museum, Wiltshire, UK

This was a relatively recent (1997) metal detectorist’s find. It has a huge rock crystal in the middle held by cross bands of gold and embellished in the centre with a blue paste or possibly (they say) lapis lazuli bead. It is a very particular object that would have been the handle of an aestel or pointer for the recitation of passages in the Bible or church liturgy. The pointer was possibly made of wood, bone or ivory (walrus for this period much more likely than elephant). These were sent to various people by King Alfred (reined 871–899), the Anglo-Saxon king credited with creating the idea of ‘England’ and possibly intended to aid reading scripture. It is called the Warminster Jewel in tribute to its find spot. Warminster is in the south of the English county of Wiltshire, a stronghold of Wessex, Alfred’s ‘home’ kingdom.

The Warminster Jewel displays a large translucent rock crystal roughly fashioned, set in an open gold amulet. The gold work simulates the much harder technique of granulation by indenting the surface to look like a beading (credit: Wikimedia Commons/user:geni, CC BY-SA 4.0 https://creativecommons.org/licenses/by-sa/4.0).

Read more: https://www.salisburymuseum.org.uk/collections/archaeology/warminster-jewel

2. The Alfred Jewel, c. late 9th century. Ashmolean Museum, Oxford, UK.

This is the more famous one because it has been known about for so long, found in Somerset, England in 1693. Somerset was also in the headlands of Wessex. It is the same kind of object and of a similar date as above but has an inscription in Anglo-Saxon: AELFRED MEC HEHT GEWYRCAN – ‘Alfred ordered me to be made’ – which is why archaeologists and historians believe it to be directly connected to King Alfred, much later designated ‘the Great’. The rock crystal here has a lot more clarity than the Warminster Jewel and forms a kind of doublet to magnify the cloisonné enamel depiction of a male (?) figure perhaps of Alfred himself. I have come across similar imagery using just this technique of cold inlay slices of coloured glass paste on a disc brooch from Lombard southern Italy, now at the British Museum, while research my PhD thesis (another reason I am fascinated by this object).

The Alfred Jewel uses rock crystal to partly encapsulate and magnify the cloisonné enamel effigy of a figure. The inscription is punched into the gold work along the perimeter of the jewel and the standard of gold work on this piece is much higher and more sophisticated than on the Warminster Jewel. The reverse of the jewel is gold engraved with foliate patterns (credit: Wikimedia Commons/user:geni).

Read more: https://collections.ashmolean.org/collection/browse-9148/object/75670

Rock crystal magic and meaning

Rock crystal is one of the most widely used magical stones and so its significance also varies according to different traditions and peoples’ practices. Crystalline quartz like rock crystal has piezoelectric (generates ‘static’ from pressure or friction e.g. being rubbed) and pyroelectric properties (naturally electrically polarised like a magnet and so can create their own physically detectable electrical fields). These properties have lent the crystal physically visible qualities that change the atmosphere around them.

I relate to rock crystal as a cleanser and an amplifier. It is especially useful when you want to reset yourself or a situation that is ongoing, to re-energies after you feel depleted, or to emerge with wisdom and clarity from a period of confusion or frustration. In Western crystal healing, rock crystal points are fundamental to the practice, used to direct energy into and out of spaces, including the body, and are frequently used in combination with other stones with more specific vibrations. The natural wand shape of rough crystals have lent them to be used as such in earth-based magical traditions such as Wicca.

There is a huge amount of lore about rock crystal (variously clear or colourless quartz) but I recommend reading Melody’s interpretation in her book: Love is in the Earth. a Kaleidoscope of Crystals. Updated edition (2003), pp. 503-6.

A giant cluster of colourless quartz points (rock crystal) with bipyramidal terminations and uneven crystal faces forming six-sided prisms on display at Canterbury Museum, Christchurch, New Zealand, 2017 (credit: Tehmina Goskar). All quartz crystallises in the trigonal crystal system.

1 comment

Comments are closed.