A grotto is “a natural or artificial cave used by humans in both modern times and antiquity” (Wikipedia). Grottos are fascinating subjects for a mineralogist – the article below deals with European grottoes, but there are also grottos of a different kind in North Africa and China.
Natural grottos are often found near water and may flood at high tide, such as the famous grottos around the Mediterranean and Adriatic Seas, often accessible by swimming or diving. Inland, grottos (or caverns) are associated with the water-eroded areas of limestone in inland areas, and this is where stalagmites and stalactites form from mobilised carbonate minerals such as calcite.
A third type of natural grotto, normally called a “lough” or a “vug”, is the most beautiful of all three types, and is usually only discovered when mined into, or when revealed by rock falls. These caverns are lined with beautiful crystals such as fluorite, calcite, barite, gypsum, galena, sphalerite, or pyrite (the spar minerals of the Victorian miners). Caverns or vugs in granitic systems may be lined with rock crystal or amethyst.
One (shortened) description of the minerals to be found in a cavern opened by mining was provided by an amazed John Williams in 1789: “There is an innumerable multitude of short prismatical crystals which sparkle like a thousand diamonds, and between these and sticking to them promiscuously, there are pyrites and spar shot also into prismatical, cubic and other figures, and clusters of grotesque figures which grow out of one another. The whole inside of the cavern is magnificently adorned with all the gay and splendid colours of the rainbow and the peacock’s tail”!
Man-made grottos over the ages have exploited natural rock systems or were constructed from scratch. They were lined with natural crystals, mirrors, coloured glass and/or shells, and many other artefacts.
Grottoes, natural and man-made, were very popular in Ancient Greek and Roman culture and invariably contained a spring, a fountain or a well, sometimes even a bathing area. Often statues of gods and goddesses featured, as well as sculpted mythological beings. Later, in the sixteenth century, artificial grottoes became popular in Italy and France, usually set in large gardens, and often incorporating “water tricks for the unsuspecting visitor”. In parts of Switzerland, grottoes were used as areas to store and preserve wine and food, some of these later became restaurants
The grotto constructed in his Twickenham garden by Poet Alexander Pope in the 1740s, and the grotto of Charles Hamilton built in Surrey in the 1760s, are fine examples of English grottos. Both still exist, although partially damaged, and are being restored by Trusts.
By the mid nineteenth century, the fashion for grotto-building in the UK had waned, but there has been a revival recently. This has seen the restoration of many older grottoes and construction of new ones. Many of these are beautifully lined with shells and corals (see tatler.com/article/theposhest-grottos)
The most lavish crystal grotto is found near Edinburgh in Scotland at Bonnington House. This is encrusted with 15 tons of Brazilian amethysts. The owners say going down into the grotto is like “being baptized”, a similar feeling of awe to that experienced by those mineralogists lucky enough to visit crystal pockets in mines, or of cave explorers finding an unexpected cavern complete with stalagmites and stalactites.