Hi Roy
Paul Ysart, who was a fluent French speaker, used to travel to Paris in the 1930's to purchase supplies of colour and adventurine, so he was using it a long time ago !
Here is a weight from my collection that dates to this very early period and is possibly a trial for the Harlequin design that was one of PY's bread and butter weights or maybe a test to see how the colours would work together. Plenty of Adventurine as you can see, and great Monart colours. The base shows Pauls typical knocking off marks.
Best regards
Derek