Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2022	Dec. 31, 2021
Current assets:
Cash and cash equivalents	$ 133,384	$ 105,355
Trade receivables, net	33,592	26,715
Inventories, net	19,579	15,116
Prepaid expenses and other current assets	3,435	3,244
Total current assets	189,990	150,430
Property, plant and equipment:
Mineral reserves and land	48,586	40,534
Proved natural gas properties, successful-efforts method	15,934	15,934
Buildings and building and leasehold improvements	9,588	7,856
Machinery and equipment	359,123	342,120
Furniture and fixtures	1,312	1,173
Automotive equipment	7,054	5,944
Property, plant and equipment	441,597	413,561
Less accumulated depreciation and depletion	(269,627)	(251,389)
Property, plant and equipment, net	171,970	162,172
Operating lease right-of-use assets	5,372	3,144
Other assets, net	440	450
Total assets	367,772	316,196
Current liabilities:
Accounts payable	7,725	5,433
Current portion of operating lease liabilities	1,411	899
Accrued expenses	6,401	4,856
Total current liabilities	15,537	11,188
Deferred tax liabilities, net	25,582	23,055
Operating lease liabilities, excluding current portion	4,129	2,311
Other liabilities	1,436	1,436
Total liabilities	46,684	37,990
Stockholders' equity:
Preferred stock, $5.00 par value; authorized 500,000 shares; none issued or outstanding
Common stock, $0.10 par value; authorized 30,000,000 shares; 6,703,166 and 6,681,469 shares issued at December 31, 2022 and 2021, respectively	671	669
Additional paid-in capital	34,528	31,774
Retained earnings	342,504	301,611
Less treasury stock; 1,021,087 and 1,015,457 shares at December 31, 2022 and 2021, respectively, at cost	(56,615)	(55,848)
Total stockholders' equity	321,088	278,206
Total liabilities and stockholders' equity	$ 367,772	$ 316,196