brittle.. i mixed white cement with coral chips (aragonate sand) to a paste form, then i form the shape of the rock i want in a box of damp sand and pour in the paste to form the shape. Leave it to dry for 2-3 days. Once dried, soak into fresh water for 5-8 weeks then its ready (do rem to change the water to avoid mossy breeding.
Want more info: go to bio ocean, justin is also doing this.