Here is a seed that got every kind of ore underneath spawn that anyone could wish for. You will be able to find more than ten diamonds, plenty of iron and gold and some coal.
Dig straight down until you come to an entrance of a cave with some lava next to you andiron right in front of you. There are more than 8 iron ores which you can mine here. Visible at the same area are some diamonds ores. First get the iron ores to craft yourself an iron pickaxe which you can use to get the diamonds ores.
To find the remaining diamonds turn around from the first diamonds so that you face the large pool of lava. Just a bit further away you should be able to see some more diamonds and gold in the ceiling and wall of the cave.
And if that wasn’t enough there are more diamonds to be found. Check out the video further down to learn how to get to the last four diamonds.