Crystal weapons in Roblox Rivals were added during Update 8 , when the Legendary Crystal Bundle was introduced. The wiki snippet also says Crystal Daggers were first available in that bundle before later being replaced in Update 14.

Quick Scoop

  • Added: Update 8
  • How they arrived: Through the Legendary Crystal Bundle
  • Later change: Crystal Daggers were replaced with Keyvolver in Update 14

TL;DR: crystal weapons entered Rivals in Update 8 via the Legendary Crystal Bundle.