free stone refers to a type of stone that is readily available or not restricted in any way.

मुक्त पत्थर

English Usage: The architect chose free stone for the building to reduce costs.

Hindi Usage: वास्तुकार ने लागत को कम करने के लिए मुक्त पत्थर का चयन किया।

stone refers to a small piece of rock or a hard mineral matter.

पत्थर

English Usage: He picked up a stone from the ground to skip across the water.

Hindi Usage: उसने पानी पर कूदने के लिए जमीन से एक पत्थर उठाया।

free means not under the control or in the power of another; able to act or be done as one wishes.

स्वतंत्र

English Usage: She offered free advice to anyone who needed it.

Hindi Usage: उसने जरूरतमंदों को स्वतंत्र सलाह दी।