Tested 25+ Everlasting Crystals in multiple biomes and only ever got 3 drops: 85% of the time it was Emerald, 10% of the time it was Diamond, and 5% of the time it was [Redacted]. Tooltip specifies the Everlasting crystal should produce a random crystal when right clicked.

Biomes don't affect what drop you get from the crystal. You got the chances about right and it can only drop emeralds diamonds and astral diamond shards intentionally.