Section 1. The weights and measures of the metric system may be employed and used in the commonwealth, and no contract or dealing shall be deemed invalid and no pleading in any court shall be open to objection because the weights or measures are stated therein in terms of the metric system; provided, that the carat weight of two hundred milligrams, and its multiples and subdivisions, shall be the sole legal standard for the buying and selling of diamonds and other precious stones. The metric weights and measures received from the United States and now in the custody of the deputy director of standards may be used as authorized standards, and shall in no case be removed from his custody except when necessary for their preservation or repair.