Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 83,562	$ 54,260
Trade receivables, net	22,979	22,948
Inventories, net	15,210	13,388
Prepaid expenses and other current assets	2,245	2,139
Total current assets	123,996	92,735
Property, plant and equipment:
Mineral reserves and land	40,065	36,423
Proved natural gas properties, successful-efforts method	15,934	17,484
Buildings and building and leasehold improvements	7,808	7,251
Machinery and equipment	318,503	304,379
Furniture and fixtures	1,088	981
Automotive equipment	4,802	3,837
Property, plant and equipment	388,200	370,355
Less accumulated depreciation and depletion	(235,739)	(219,668)
Property, plant and equipment, net	152,461	150,687
Operating lease right-of-use assets	2,226	3,192
Other assets, net	415	423
Total assets	279,098	247,037
Current liabilities:
Accounts payable	4,592	4,430
Current portion of operating lease liabilities	1,187	1,294
Accrued expenses	5,809	3,735
Total current liabilities	11,588	9,459
Deferred tax liabilities, net	21,531	17,218
Operating lease liabilities, excluding current portion	1,030	1,866
Other liabilities	1,757	1,362
Total liabilities	35,906	29,905
Stockholders' equity:
Preferred stock, $5.00 par value; authorized 500,000 shares; none issued or outstanding
Common stock, $0.10 par value; authorized 15,000,000 shares; 6,657,880 and 6,627,000 shares issued at December 31, 2020 and 2019, respectively	666	663
Additional paid-in capital	29,457	27,464
Accumulated other comprehensive loss		(1)
Retained earnings	268,186	243,566
Less treasury stock, 1,009,796 and 1,004,174 shares at December 31, 2020 and 2019, respectively, at cost	(55,117)	(54,560)
Total stockholders' equity	243,192	217,132
Total liabilities and stockholders' equity	$ 279,098	$ 247,037