I first came across reference to hydrogrossular garnets when gathering information online about the stones that could be found on Gemstone Beach, Orepuki, on the south coast of New Zealand’s South Island. Some brief descriptions of Gemstone Beach include comments along the following lines:  “Semi-precious gems such as garnet, jasper, quartz and nephrite can often be found on the beach. A few hours beachcombing could easily yield gems such as hydrogrossular, jasper, fossil worm casts and the elusive sapphire” (Source: Information about the Riverton–Aparima South Coast Heritage Trail. See also Nature’s Edge: Tuatapere and the mindat.org entry on Gemstone Beach.) The source of this could be a Heritage Trail sign that, to my knowledge, is no longer at Gemstone Beach (I don’t recall having seen it on any of my visits).

Source: https://talltales.me/2013/02/24/the-south-of-the-south/photo-2-01-13-10-30-21-am

The Encyclopedia of New Zealand mentions hydrogrossular garnets in the following terms:

Calcium-rich garnet is…