Disclosed is an optical fiber formed from a preform that includes a clad material and a core material. The clad material includes an oxide glass and the core material includes as a primary component a semiconductor material that melts at a temperature below where the cladding glass draws into fiber. In addition, the core material includes a scavenger for oxide precipitates that can precipitate into the core from the adjacent clad material during the fiber formation process. During formation, the scavenger reduces oxide precipitate to form the core primary component, or a phase compatible with it, and volatile products. The volatile products evolve out of the system. The primary material of the core can melt during the fiber formation process and crystallize upon cooling leaving the formed optical fiber with a crystalline core and little or no impurities due to precipitation of oxide into the core.