Consolidated Balance Sheets - USD ($) $ in Thousands	Dec. 31, 2023	Dec. 31, 2022
Current assets:
Cash and cash equivalents	$ 187,964	$ 133,384
Trade receivables, net	38,052	33,592
Inventories	24,313	19,579
Prepaid expenses and other current assets	4,640	3,435
Total current assets	254,969	189,990
Property, plant and equipment:
Mineral reserves and land	59,307	48,586
Proved natural gas properties, successful-efforts method	15,934	15,934
Buildings and building and leasehold improvements	10,732	9,588
Machinery and equipment	374,000	359,123
Furniture and fixtures	1,312	1,312
Automotive equipment	8,313	7,054
Property, plant and equipment	469,598	441,597
Less accumulated depreciation and depletion	(289,803)	(269,627)
Property, plant and equipment, net	179,795	171,970
Operating lease right-of-use assets	5,273	5,372
Other assets, net	565	440
Total assets	440,602	367,772
Current liabilities:
Accounts payable	7,404	7,725
Current portion of operating lease liabilities	1,582	1,411
Accrued expenses	8,505	6,401
Total current liabilities	17,491	15,537
Deferred tax liabilities, net	24,659	25,582
Operating lease liabilities, excluding current portion	3,919	4,129
Other liabilities	1,429	1,436
Total liabilities	47,498	46,684
Commitments and contingencies (Note 8)
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; 6,731,207 and 6,703,166 shares issued at December 31, 2023 and 2022, respectively	674	671
Additional paid-in capital	37,820	34,528
Retained earnings	412,499	342,504
Less treasury stock; 1,026,651 and 1,021,087 shares at December 31, 2023 and 2022, respectively, at cost	(57,889)	(56,615)
Total stockholders' equity	393,104	321,088
Total liabilities and stockholders' equity	$ 440,602	$ 367,772