It is easy to a crystal if you know how to creat a rock actually, not so much difference ...
what your gonna change is the material itself only which is (1-Mix Shader 2-[shader-1:Glass]~[Shader-2:Valvet] make the Valvet low in Sigma about 0.300) and then you can change the colors of the materials u made until you see a good fitting color,+Since it just a glass then you'll need some lighting i actually used those ~Tools~ to create it perfect!
1-Use the lamp into sun and direct it to the Mesh you created (Crystal), if you don't know how then checkout some tutor..
2-You'll need some other light source so make sure you have a plane and emission it about 4-5.000 (as you like but i recommend 3-4.500)
3-As well as Floor you need i decided to make it on a volcanic cave, checkout Andrew channel in YouTube he know how to make a REal Cave
4-You also need a background i made this : [Create a plane at the background or Dublicate the plane you created for the floor+Make the world 100% blak +...
