Yup. It's a stamp. More specifically, a tre skilling gul or Treskilling Yellow.
It was printed in 1855. The treskilling (3-skilling) stamp was supposed to be blue-grey, but, due to a printing error, an unknown number were yellow, the colour of the 8-skilling stamp. No-one seemed to notice this at the time, until a stamp collector by the name of Georg Wilhelm Backman found one in 1886, and bought it for 7 kronor - by a very rough calculation, about £80 or $120 dollars in today's money.
No other surviving treskilling yellows have been found, and it sold in 2010 for $2,060,000 to a Msr Arman Rousso, a French business tycoon. Given that a stamp ways quite a bit less than a gram, this puts it at at least $2,000,000/g, making it more than 37,000 more valuable than gold in terms of weight.
Go, three60!