Complex sentence: One milestone was the discovery of the geometrical law of crystallization by René Just Haüy (a further development of the work by Nicolas Steno and Jean-Baptiste L. Romé de l'Isle), another milestone was the notion that metals are elements too (Antoine Lavoisier).
Split into simpler sentences: One milestone was the discovery of the geometrical law of crystallization by René Just Haüy (a further development of the work by Nicolas Steno and Jean-Baptiste L. Romé de l'Isle). Another milestone was the notion that metals are elements too (Antoine Lavoisier).