TumbleStone Calendar 2026: December – Hydrogrossular Garnets

The Introduction to this Calendar can be found here. 

The final month in the 2026 TumbleStone Calendar features five hydrogrossular garnets. They were all found on the Te Waewae Bay coast in Southland, four on Gemstone Beach and one a few kilometres west at Papatotara. In general, a variety of stones are known as hydrogrossular, some being translucent, some opaque. Colours also vary, depending on what other minerals are present. Hydrogrossular garnets usually feel waxy and they shine brightly when wet on the beach. “Grossular” comes from the botanical name for the gooseberry, and gooseberry green is one of the classical colours of a more translucent type. Hydrogrossular garnets are not crystals but occur as rock masses. They are probably the most sought after stones on Gemstone Beach, often being made into pendants. Maori used to use them as hammer stones, as they are heavier and harder than most other stones. Hydrogrossular garnet is one of 13 minerals first described from New Zealand and accepted as valid by the International Mineralogical Association – New Zealander Colin Hutton analysed some Nelson stones in 1943 and noted their uniqueness. The Nelson and Southland hydrogrossular garnets once existed side by side, but movement associated with the Alpine Fault has now separated them by some 400 kilometres. For more, see “Hydrogrossular Garnet” and the various links listed at the end of it.

Stone One (top row, left, in Calendar) was found on Gemstone Beach on 6 March 2025, a day when paragliders appeared over the beach: xx

The green in this stone is reminiscent of hydrogrossular’s typical translucent gooseberry green but there are other minerals present, providing the white and brown clouds.

The green colour of Stone Two (top row, right, in Calendar) is a great example of the classic gooseberry-green hue of a hydrogrossular, and is a deeper green than most specimens. I came across it on Papatotara Beach on 15 August 2025, the furthest west on Te Waewae Bay that I have found one:

It is not as smooth as some specimens but it is bigger than most I have found. I count it among the best half dozen hydrogrossular garnets I have picked up.

Stone Three (bottom row, left, in Calendar) is another Gemstone Beach hydrogrossular garnet, found on 20 March 2025. I found it near the cliffs at the back of the beach – even dry, these stones stand out:

I take photos of unpolished stones while they are wet so that their details show. Water tends to bead off the waxy surface of hydrogrossular garnets. The small wet patch at top of Stone Three, just to the right of centre, also reveals its waxy character.

On 25 August 2025, I found quite a number of nice hydrogrossular garnets on Gemstone Beach – this was in fact, at the time of writing, the most recent time I had fossicked there, though I hope to return in March 2026. On that day in August, I found both Stone Four and Stone Five. Stone Four (bottom row, centre, in Calendar) is an example of the attractive opaque tan coloured variety of hydrogrossular garnet, with some bright yellow along the left-hand side:

Stone Five (bottom row, right, in Calendar) is similar to Stone Three, a white garnet though a little more tansparent and with some larger bright white clouds in it giving a sense of depth.

It will soon be time to put up the 2027 Calendar!