Generally speaking these watercolour paint cakes are not common when considered along with paint in pans. 
However this particular one has at least two more levels of rarity!
Firstly this was made by Rowney And Forster. All items made by these makers are extremely rare. 
The name came about because of a partnership that was entered into in 1815 and lasted until 1831.
The two partners were George Rowney and the brother of George’s wife Esther Forster Richard. 
The partnership was dissolved because of Richards old age. 
They were a very successful company who produced beautiful things. 
Secondly the thing that makes this item so rare is that it is made from genuine Indian Yellow which is in my opinion one of the most beautiful of pigments although always very rare. 
The thought of combining these two aspects is very interesting to me and certainly is the only one I’ve seen. To be sure other colours by this maker do turn up but I’ve yet to see another like this. 
If all of this is not enough just look at its condition it might have been made yesterday!However I suspect  it must have been kept covered in a box since it was made all those years and lifetimes ago. 
Practically speaking this colour is very special it to all intents and purposes looks dull however if it is touched by water it literally bursts into life and always reminds me of the Sun coming from behind dark clouds, just glorious!
Approximately,
3cm
1.5cm