I need to create a method for a weighted random function

I need to create a system where something will be randomly generated but with more objects being spawned closer to the max number.

Specially I would like to create a sort of mining game. I want to have all the gems spawned randomly underground but as you go deeper, gems become more common.

Kinda like this image

I have no problem spawning these gems at the X position. That will just be a random value of screen width. Its the Y value I am stuck on. For example, shiny rocks can be found at y=0 through y=100. I would like to randomly spawn say 30 rocks at random heights but somehow weight the randomness so that more rocks appear closer to y=100.
A Chart like this