This cache has been located within St. Sebastians Cemetery Wokingham Without with the kind permission of Wokingham Borough Council. The Council has asked me to remind visitors that this cemetery is still active, there are funerals still taking place and mourning relatives who visit on a regular basis. Also the ground can be uneven and visitors should be careful when walking around especially between the graves. Vehicular access is available from Heathlands Road.
Since the cache is an Earthcache, there is no physical container to search for at the coordinates or around the grounds of the cemetery. Instead, you will need to look at the geology of the rocks that have been used to form some of the gravestones and answer a few simple questions set out below.
Cemeteries are fantastic easily accessible resources for anyone interested in geology and rocks. The local geology around Wokingham mainly comprises clay overlain by sands and gravels but other rocks from all over Britain and beyond can be found in the cemetery.
The coordinates will take you to a group of gravestones which from a distance appear dark grey in colour but close up you will see that they have a very unusual blue iridescence.
The rock used for these gravestones is known as larvikite and originates from Norway. It was formed some 250-300 million years ago. Like granite, larvikite is an igneous rock that is crystallized from magma slowly cooling below the surface of the Earth. However, unlike granite which contains a lot of quartz, larvikite is mainly comprised of feldspar, in fact larvikites are generally composed of 80–95% feldspar. The feldspar content of larvikite comprises both alkali feldspar (sometimes known as K or potassium feldspar) and plagioclase feldspar. The larvikite is formed in microscopic alternate layers of alkali and plagioclase feldspar known as lamellae.
Normally feldspar can be found in various colours such as pink, white, grey or brown but the feldspar in larvikite gives rise to an interesting property when viewed from different angles. Look at the larger crystals on one of the gravestones and move your head to look at it from different angles. The iridescent colour should change when the viewing angle is changed and become a lovely shimmering blue colour. This blue colour or iridescence is caused by light passing from one alternating lamella or layer of crystal to another and the effect is basically caused by light refraction within the layering. The layering is on a comparable scale to the wavelength of blue light and the layering scatters this wavelength to give the crystals a blue appearance. This differs from colours due to pigments or preferential scattering where the colour does not change with changes in the viewing direction. The intensity as well as the colour of the iridescence depends on the spacing and geometry of the lamellae. The phenomenon is best seen on a sunny day and is called the "Schiller effect.
In general, larvikites displaying strong blue play of colours or iridescence are more attractive and valued than the ones with weaker blue and/or silvery colour-play. Commercially the more intensely coloured larvikites are used extensively for worktops and shopfronts as well as gravestones and have been given names such as “blue pearl”, “blue antique”, “emerald pearl” and “silver pearl”. Larvikite is extracted in rectangular blocks aimed for export markets. However, if the blocks contain flaws, veins, discolouration or other features reducing the uniformity of colour and quality, the market price for the blocks is reduced significantly, if they can be sold at all. As a result the average block yield in the larvikite quarries is only about 10% which means that nine out of ten cubic metres of extracted rock are not utilized for commercial purposes. However the leftover rocks are to an increasing extent used for other purposes particularly in Britain.
To log this earthcache, please email me the GC number and the cache title together with answers to the questions below. The given coordinates will allow you to recognise the larvikite but the first and second questions can be answered by observing any larvikite gravestone within the cemetery.
1 Look at any of the larvikite gravestones and focus on one of the larger crystals. Move the angle of your head and note the change in the colour. Why does this change occur? (Hint: The answer is in the text)
2 Look at any of the larvikite gravestones. What is the largest size of crystal with the iridescent effect? (Hint: If you do not have any means of measuring the size of the crystal, then mark its outer edges on a scrap of paper or even the back of your hand and measure it when you get home)
3 Look at the grave stone in memory of David Arthur Kingdon which is within 5m of the given coordinates. This gravestone has an unpolished top. Is the iridescent appearance more apparent on the polished or unpolished surface?
4 Commercially high grade larvikite is commonly used in the home, the High Street and in cemeteries. What is commercially inferior grade or waste larvikite used for in Britain? (Hint: This use occurs in coastal counties but not Berkshire)