goldsmith Bartholomeus Jansz van Assendelft with octahedron diamond ring and touchstone

Bartholomeus Jansz van Assendelft

This painting by Werner van den Valckert is on display at the Rijksmuseum in Amsterdam. It shows a portrait of Bartholomeus Jansz van Assendelft (Leiden, 1586-1659). He is leaning out of a window, in his right hand he holds a ring with a large octahedron and in his left hand he holds a touchstone. Both objects are clues that point to his profession as a goldsmith. Van Assendelft’s left hand is brightly lit, which draws your attention to the touchstone, which also shows off his honorable function as judge of his guild. And on the touchstone the painter signed his work: We read W v Valckert fe 1617.

Werner van den Valckert

Van den Valckert was a mysterious person in history and his name is not found in the registers after 1627. He became a member of the painters Guild of St. Luke in the Hague between 1600-1605. From this we can assume that he was born around 1580-1585. We can conclude that by 1614 he had moved to Amsterdam, because his daughter was baptized there. His earliest dated prints are from 1612. His surviving paintings are historical allegories and portraits. Van den Valckert also made a prestigious schutterstuk, which features the Amsterdam major Albert Burgh. According to his biographer Arnold Houbraken, Van den Valckert was a student of Hendrik Goltzius.


A touchstone is a small tablet of dark siliceous stone (such as fieldstone, slate, or lydite) used for assaying precious metal alloys. It has a finely grained surface on which soft metals like gold, leave a visible trace. Assaying by touch was one of the earliest methods used to measure gold alloys. You draw a line with gold of your jewel on the stone. This can leave a scratch on the jewel if you have to remove any upper gilt layers. Alongside the drawing you make another scratch of known gold samples. Then the traces of gold are treated with acids that dissolve impurities. The trace will react differently to specific concentrations of nitric acid applied, and by this you measure the gold content of the jewel. The color of the reacted area is compared to that of the reference sample. A 14 carat (or any lower carat) gold jewel will show chemical activity and dissolve when tested with 18 carat gold acid, but when the trace is not affected it can be identified as 18 Carat gold (this means 750/1000 gold and 250/1000 other materials; copper, nickel, zinc).

A complicated detailed operation to achieve a beautiful result!

Octahedron ring and the meaning of adamas

An octahedron is one of the diamond’s natural crystal shapes. And the ring that Van Assendelft is holding clearly contains an octahedron cut stone. We see a piece of glass or a rock crystal, but most likely a diamond. Diamonds crystallize as octahedrons, cubes, or dodecahedrons. Try to scrabble that! It sometimes shows habits that contain two or more of these forms. But the octahedron is one of the rarest forms.

16th century Moghul ring with octahedron diamond

The word diamond originates from adamas which means invincible in Greek. The first adamas came from India and were hardly cut. Because of their beauty and strength, they were worshiped as talismans, and cutting a diamond would not benefit its strength. The Europeans changed this point of view, in order to bring out the fire and brilliance of diamonds. Symmetrical octahedrons, very rare rough crystals, were the first to be polished or cut – in a pyramidal diamond like the one in van Assendelft’s ring. One pyramid is completely hidden in the shank of the ring, the top part is shown.

Quite fantastic to look at and even more to carry! I also love this 16th century Moghul ring with a cinnamon octahedron diamond that we sold a few years ago.


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