Chalk is a soft, white, porous, sedimentary carbonate rock. It is a form of limestone composed of the mineral calcite and originally formed deep under the sea by the compression of microscopic plankton that had settled to the sea floor. These are cut from natural limestone in India, natural chalk stone pencils. Crunchy, thin, earthy, becomes creamy. Brittle and can break easily but, great to write with on slate and chalk boards. A slate pencil was used to write on the slate board. It was made from a softer and lighter coloured stone such as shale or chalk. Usually, a piece of cloth or slate sponge was used to clean it and this was sometimes attached with a string to the bottom of the writing slate.