Page 3 – Other gemstones
Almandine garnet is an iron-rich variety that appears pink or red and is sometimes mistaken for ruby. It is found in many rocks on the West Coast. Larger almandine garnets are almost always too flawed for use in jewellery. Tiny crystals of almandine garnet are abundant in sand along the West Coast, often giving it a pinkish hue.
Calcium-rich garnet is called grossular. A red form, found in South Westland, is known as hessonite. Another variety, containing some water, is called hydrogrossular and was first identified at the Roding River near Nelson. It is also found on the beach near Orepuki in Southland. Rounded lumps of pale green hydrogrossular take a good polish and have been used for jewellery.
Hydrogrossular pebbles, being heavy and exceptionally hard, were used by Southland Māori as hammer stones for the making of stone implements.
Pink manganese minerals
Rhodonite, a pink manganese silicate, is found as lens-shaped deposits within schist in Otago and along the western side of the Southern Alps. It usually forms boulders with a rind of dense black oxidised material called pyrolusite.
Fine-grained mica and quartz accompany pink piedmontite in schist that is used as a decorative stone in Central Otago. When tumbled the mica grains tend to drop out of the schist, but it can produce a sparkling gemstone.
Goodletite is a local name for a rare and distinctive greenish-grey ruby rock – a form of the mineral corundum. This is found only near Hokitika on the West Coast. Despite more than 100 years of searching, goodletite has been found only as boulders and never in the place where it was formed. The composition of the rock suggests it comes from lens-shaped deposits of the mineral serpentine within schist, on the western side of the Southern Alps. Although rare, blue corundum has been found in Nelson and Southland.
Kyanite, an aluminium silicate, is one of the few blue minerals that occur naturally. Beautiful in jewellery, it is difficult to polish – and even harder to find. It occurs within schist, often in association with green chrome-rich mica, in a remote part of South Westland. Rare pebbles of blue kyanite schist can be found on the beaches between Jacobs River and Hunt Beach in South Westland.
Obsidian is dense, black volcanic glass that forms on the edge of rhyolite lava flows which have been suddenly cooled in contact with air or water. Good quality obsidian breaks with smooth, conchoidal (curved) surfaces, forming sharp blades that can be used for cutting. Obsidian is known from several localities, but the best material, much prized by Māori, comes from Mayor Island.