You might want to know what you're talking about before you post.
Originally Posted by Zuu
As far as I know, since it is a prismatic gem, one Nightmare Tear alone should cover all three requirements for the meta (that is, it should count as a blue, red, and yellow gem).
If only it worked like that! Unfortunately the gem is only counted as active for the colour of socket you place it in. So for example, on a helm with a meta + a yellow socket, placing the Tear into the yellow socket will result in a count of one yellow gem active. That's all well and good, but that still leaves you with the blue to match, and you're probably better off using the Tear there instead as it's of greater benefit than any of the alternative purple gems, and pair it with a +hit/+str yellow in the helm's socket.
This is the way prismatic gems (e.g. pre-3.2 Dragon's Eyes) have always worked.