INCOME TAXES - Deferred Tax Assets and Liabilities (Details) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 30, 2017	Dec. 31, 2016
Federal:
Current			$ 988,000	$ 613,000
Deferred			27,000	1,284,000
State:
Current			32,000	2,000
Deferred			(26,000)	249,000
Total			$ 1,021,000	$ 2,148,000
Difference between tax rate on income for financial statement purposes and federal statutory tax rate
Statutory tax rate (as a percent)		34.00%	34.00%	34.00%
Remeasurement of deferred tax liability, provisional benefit			$ 27,000
Percentage depletion (as a percent)			(4.70%)	(1.80%)
Non-deductible expenses (as a percent)			1.70%	1.00%
Valuation allowance for tax assets (as a percent)			2.50%	1.00%
State income taxes, net of federal benefit (as a percent)			2.00%	4.20%
Domestic production deduction (as a percent)			(0.30%)	(0.80%)
Revaluation of deferred items for new Tax Act (as a percent)			(1.00%)
Other (as a percent)			1.80%	(0.80%)
Total (as a percent)			36.00%	36.80%
Current statutory federal and states tax rates (as a percent)			25.74%	37.96%
Deferred tax assets
Reserves for self-insured losses			$ 328,000	$ 579,000
Accrued reclamation			1,386,000	1,945,000
Unfunded supplemental profit sharing plan liability			275,000	351,000
Asset valuation reserves			293,000	267,000
Future state tax credits			863,000	789,000
Net state operating loss carryforwards			126,000	167,000
Federal AMT carryforward			620,000	620,000
Other			701,000	873,000
Deferred tax asset before deferred tax liabilities and valuation allowance			4,592,000	5,591,000
Deferred tax liabilities
Depreciation			859,000	1,013,000
Deferred development			1,298,000	1,057,000
Prepaid royalties			161,000	1,187,000
Other			458,000	588,000
Total deferred tax liabilities			2,776,000	3,845,000
Net deferred tax asset before valuation allowance			1,816,000	1,746,000
Beginning balance		$ (130,000)	(130,000)	(73,000)
(Increase) decrease during the period			(70,000)	(57,000)
Ending Balance			(200,000)	(130,000)
Net deferred tax asset			$ 1,616,000	$ 1,616,000
Scenario Forecast
Difference between tax rate on income for financial statement purposes and federal statutory tax rate
Statutory tax rate (as a percent)	21.00%