INTRODUCTION TO HYDROGROSSULAR GARNETS: Unlike the garnets we know as precious gems, hydrogrossular garnets come from dense rock masses, not crystals. Technically, hydrogrossular garnets are a calcium aluminium garnet with hydroxide partially replacing the silica found in other garnets. The first ever identification of hydrogrossular garnet in the world was in 1943 by Colin Hutton, from stones found in Nelson, and it is the most widely spread of the 13 minerals first described from New Zealand. Hydrogrossular stones (or, to be more accurate, stones that have some hydrogrossular garnet in them) often feel waxy and some are partly transparent. “Grossular” derives from the Latin word for gooseberry, referring to the light-green colour of some hydrogrossular garnets. However, other minerals get mixed in, and white and brown are two other common colours of hydrogrossular stones. Hydrogrossular stones are easily polished and make excellent pendants.
In her book “The New Zealand Rockhound” (1981), Natalie Fernandez has an entry on “Grossular Garnet” on page 50 (see photo above, left): Grossularite or hydrogrossular garnet is a massive form of calcium garnet…Colour: A wide range, patchy. Pale green, greyish, yellow, white, brown. Occasionally shades of orange, darker green, even blue and pink. Opaque to translucent. Lustre: Waxy, vitreous. [Note the treatment of hydrogrossular garnet and grossular garnet as relating to the same type of stone, something sometimes done by authors. I am not yet clear on the significance of the differences between the two.] On page 34 of “A Photographic Guide to Rocks and Minerals of New Zealand” by Nick Mortimer, Hamish Campbell and Margaret Low (2011) is an entry on “Hydrogrossular” (see middle two photos above). Its association with serpentinite is noted, and its presence in rodingite. Their entry on “Rodingite” states: “The main minerals in rodingite are calcium-bearing hydrogrossular garnet, plagioclase and actinolite.” In Thornton’s pages on “Garnet” in her booklet “Gemstones”(1985) (also available online), she refers to a hydrogrossular stone with a dark crystal of diopside in it (see photo above, right). Kirk Feral notes that hydrogrossular garnets often contain “black inclusions of Chromite”. Certainly, Gemstone Beach specimens often have dark inclusions, some black, some grey, some silvery.
Hydrogrossular garnets are probably the most sought after stones on Gemstone Beach. I often find it hard to walk past even the tiny ones, partly because they shine so brightly. To find a larger one is always a thrill. But I still have to check each possibility carefully, as some turn out to be quartz and others turn out to be argillite – the angle of the sun can make identification deceptive. Most hydrogrossular garnets have a waxy feel, and they lack the crystalline character of quartz – it takes a while to develop a feel for their subtle differences. I am grateful to those fossickers I have met on Gemstone Beach who have shared their knowledge with me, some of whom have even given me specimens they have just found. And seeing stones in museums and private collections has also been very important in helping me to spot stones on the beach.
TYPES OF HYDROGROSSULAR GARNETS: The following are the types of hydrogrossular garnets that I have come to recognise on Gemstone Beach. This is not a systematic or exhaustive classification, just a basic idea about varieties that I personally have compiled, based on my own observations. The differences will no doubt be due to the incorporation of other minerals to varying degrees. The first type is the translucent light green stone, with little or no other material in it – sometimes it can be a deeper green, maybe even a blue/green. This in many ways is the “classic” hydrogrossular.
Second are similar stones but of different hues, especially grey. Some of these translucent and partly-translucent stones have interesting cloud-like patterns in them.
The third type is the brown hydrogrossular garnet, usually opaque and very waxy, sometimes quite dark in colour, sometimes much lighter. These are my favourite type. They don’t need to be polished to be well worth displaying or wearing.
Fourth are the opaque stones of a variety of colours, white, green, grey, brown, yellow. There is quite a diversity of these, and one stone can contain a number of colours.
Fifth are the white stones with dark “inclusions”, probably diopside, one of the most common types. There seems to be a reasonable uniformity in the white component, though some have a blush of pink in them (maybe attributable to the mineral thulite). The dark “inclusions” do not usually take a polish.
The sixth type is what I call the “porcelain” type – an opaque stone that looks and feels like porcelain china, often having fine blue-green veins. I initially did not think this was hydrogrossular garnet. I have checked with rockhound groups online, and a number of people have identified it as including hydrogrossular garnet.
The seventh type is chalcedony-like, kind of glassy, its surface often rough and broken. It has a subtly different glassy appearance than Types 1 and 2. I initially thought this was not hydrogrossular but again have been advised by more than one source that it is.
TUMBLE POLISHING HYDROGROSSULAR GARNETS: I tend to treat them lightly, often doing only one 400 grit tumble before polishing. This leaves them shiny and reveals clearly the clouds and internal patterning. However, it leaves some holes, occasional small rough patches and lines etc. Tumbling a barrel of hydrogrossular garnets produces a very fine light-coloured slurry. I often add small hydrogrossular garnets to barrels of larger stones, to assist with the tumbling process (increasing the surfaces of contact within the barrel). I often keep one or two rough hydrogrossular garnet stones with a nice waxy feel as “touch stones”, especially the translucent green ones and the rich brown ones.
NOTE ON IDENTIFICATION: The above information about hydrogrossular garnets, and the presentation of examples of these stones from my collection, is shaped by the sources I have access to. These include a small number of published and online sources, other fossickers I’ve met on Gemstone Beach over the past five years, members of New Zealand Facebook Rockhound Groups, and specimens I have seen in museums and private collections. There are two things that have made me realise how tentative the identification of these types of stones can be. First, sometimes “hydrogrossular” and “grossular” are terms used by different people for what seem to be the same kind of stone. But, as Minerals.net suggests, “Hydrogrossular is often regarded as a variety of Grossular, but it is scientifically regarded as two distinct individual mineral species that have a slightly different chemical formula than true Grossular.” Secondly, there seems to be some disagreement even amongst “experts” over whether a stone is a hydrogrossular garnet or not. For example, in August 2020 I posted a photo of a Gemstone Beach stone online, suggesting it might be a hydrogrossular garnet but noting that I wasn’t sure. Someone in North America who had undertaken technical analysis of “hydrogrossular” garnet stones from New Zealand stated that most of them were in fact “grossular” and that my stone was most likely “grossular”. But in the same discussion thread, a geological research technician from a New Zealand university had no hesitation in stating “It’s a ‘hydrogrossular’ for sure”, as did a number of amateur rockhounds familiar with Gemstone Beach.
So the information and stones presented here are basically reflective not of reliable technical scientific analysis but of “local knowledge”, informed as best it can be by a range of sources, but ultimately tentative and fallible.
Posts on TumbleStone Blog about hydrogrossular garnet:
- “What are Hydrogrossular Garnets?” – First in a series of 4 Posts. In this one, I describe my initial struggles to identify these stones and discuss some basic information about them. May 2019.
- “Why are Hydrogrossular Stones Called Garnets?” – Second in the series of 4 Posts. Presents some more technical information. May 2019.
- “Why are Hydrogrossular Garnets found in Nelson and Orepuki?” – Third in the series. Considers the role of the Alpine fault in the distribution of hydrogrossular garnets. May 2019.
- “What is Hydrogrossular’s Place Among the Thirteen Minerals First Described From New Zealand?” – Fourth and last (so far) in this series. Summarises each of the 13 minerals. May 2019.
- “Stay-at-Home Day Eleven, Sunday 5 April 2020: Stone Eleven” – A green hydrogrossular garnet, with some general information about the type of stone.
- “G” is for “Green Hydrogrossular Garnet” and “H” is for “Hematite Jasper” – A pale green hydrogrossular garnet is featured, the classic colour. June 2021.
- “January 2022, Stone of the Day #20 – Brown Hydrogrossular Garnet from Gemstone Beach” – Includes photos of hydrogrossular garnets from a veteran Gemstone Beach fossicker.
- “Stay-at-Home Day Twenty-Two, Thursday 16 April 2020: Stone Twenty-Two” – Stone 22 is a hydrogrossular garnet of light grey colour with white clouds within it.
- “February-March 2022 Fossicking Trip: Stone of the Day #15, Hydrogrossular Garnet from Gemstone Beach” – Ten different hydrogrossular garnets are considered.
- “February-March 2022 Fossicking Trip: Stone of the Day #19, Big Gemstone Beach Hydrogrossular Garnet” – This stone is maybe the largest hydrogrossular garnet I have personally found on Gemstone Beach. The Post also includes photos of other hydrogrossular garnets found that day.
- “Another South Island Fossicking Trip, February/ March 2021 – Days 17 and 18 (Gemstone Beach, Tsunami Alert)” – At the end of this Post are photos of six hydrogrossular garnets found that day.
Other TumbleStoneTwo Pages on Hydrogrossular Garnet:
HYDROGROSSULAR GARNET: PHOTOS, PART ONE
HYDROGROSSULAR GARNET: PHOTOS, PART TWO
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