National Stone Centre

Opening Hours
10am - 4pm Every day
Published 15/04/2022

Stories of Stone: Easter from the Rock Shop

Each month, NSC Volunteer, Vivienne Smith, will be providing a background into various stones and fossils that can be found in the rock shop.

Hello everyone, my name is Viv! Some people may consider my monthly features to be a load of spheroids. Well, I’m about to prove you right. This time of year one’s thoughts tend to gravitate towards Easter eggs. So I’ve been contemplating rounded objects myself.

In the Rock Shop we sell all kinds of spherical items which have been specially crafted that way. For instance, the ever popular gemstones have been tumbled to create their characteristic smooth pebble shapes. Also a hit with customers are the decorative eggs and balls of various sizes, which are sculpted from different minerals. Mind you, these elicited a rather unexpected response the other week.

While their dad was queuing for coffee, two young brothers came into the shop. They had fun investigating the stock together, discussing items of interest in hushed, secretive tones. Suddenly, on spotting the round ornaments, one of them broke the silence and declared out loud: “Oh look – balls!” For a split second you could have heard a pin drop. Then came a horrified whisper from his sibling: “You mustn’t say balls! You’re supposed to call them spheres”.

A less risqué object can be found in one of our display cabinets. It is a bunch of grapes fashioned from pieces of onyx. Perhaps surprisingly, quite a few minerals occur naturally in a fairly similar form. The shape is called botryoidal, meaning (you guessed it) bunch of grapes.

Of those we stock, rhodochrosite is one such example. It has a pretty, rose-pink colour thanks to the metal manganese. Often cut and polished ornamentally, the mineral also serves as a major ore of the metal. However, in future the deep ocean may prove to be a far more valuable source.

On the seabed, where hot water from volcanic springs meets the cold seawater, marbles of manganese form. They often develop around a tiny grain, much like a pearl grows in an oyster. But the process is somewhat slower. Each tiny ball increases in diameter by one centimetre . . . . . in around a million years!

Another mineral that occurs in a mass of small spheres is boracite. It is rich in the element boron. One of those little-known trace elements, boron is essential in you diet. Without it you would literally fall to bits, for it helps the body to use calcium to build up healthy bones. That said, chewing on boracite is probably not the best way to get your daily intake.

The botryoidal form is also a feature of azurite. Contrary to popular belief, the name does not come from the local dialect for “Are you feeling well?”. It originates instead from the Ancient Persian word for blue.
Many such early civilisations learned how to grind down minerals to create a vast array of colours for their art. From azurite they developed a rich pigment called azure blue. This became particularly popular with Renaissance artists such as Michelangelo, Leonardo, Raphael and Donatello (also known as the Teenage Mutant Ninja Turtles!)

Some minerals are found in even larger, more gently rounded shapes referred to as mammillary.

Please don’t ask me to explain – use your imagination. I’m in enough trouble as it is with the spheroids!
And before you ask – no we don’t have any in stock!