26 CFR 1.167(a)-2: (a)-2 Tangible property.

The depreciation allowance in the case of tangible property applies only to that part of the property which is subject to wear and tear, to decay or decline from natural causes, to exhaustion, and to obsolescence. The allowance does not apply to inventories or stock in trade, or to land apart from the improvements or physical development added to it. The allowance does not apply to natural resources which are subject to the allowance for depletion provided in section 611. No deduction for depreciation shall be allowed on automobiles or other vehicles used solely for pleasure, on a building used by the taxpayer solely as his residence, or on furniture or furnishings therein, personal effects, or clothing; but properties and costumes used exclusively in a business, such as a theatrical business, may be depreciated.